Because it is incorrect: it is dialect, not Standard English. "Sat" is the past tense: "he sat" or past participle: "he was sat (upon) by (someone). The imperfect tense is "he was sitting/seated".
You cannot change "he was laughing" to "he was laughed" either. 'He was laughed at' is another thing altogether. "He was seated" or "he was sitting" are standard English, "he was sat" is dialect. It is ungrammatical.
Goofy's point is that it is fine for teachers to encourage common usage of English even when incorrect. How then are their pupils to know what correct forms are? There are places around the world where language use is a really major thing for schools to concern themselves with. Yesterday there was a report of a school in Belgium where French-speaking children are obliged to speak Flemish at school on pain of punishment as that is the school policy (in a school where 40% are naturally French speakers, 60% Flemish). The riots in South Africa in 1976 which triggered the steps to the dismantling of apartheid were to do with inflicting Afrikaans-medium lessons on kids who wanted to do it in English or native African languages. That was another dimension to the argument, but an illustration of how important it is.

Sure, we have dialect forms of English used widely throughout the UK. Are they all to be embraced as "official" languages? The BBC used to follow a policy of a standard accent and standard English, but now promotes regional accents. But they do not yet go as far as inviting their correspondents to use regional dialects. Richly entertaining though they are, they are not conducive to mutual understanding.

An example: In Dundee spoken English is commonly performed without the use of any consonants whatever, just vowels, apostrophes and glottal stops. I doubt if the teachers there encourage their pupils to work in this medium, as if they were to do so their pupils' employment prospects beyond the city would be very limited. Even more importantly, the teachers would not be educating their pupils properly.

That is why British English teachers should not use dialect forms of English such as "stay sat" in a British English classroom. They shouldn't say "Yup" or "Yah" or "Nope" or "Hiya" either. These terms would not go down well in the kids' first job or academic interviews now, would they, to be practical? If the teachers know better they should show their pupils the way, and if they don't know better they shouldn't be teachers.

Yes Goofy, I know. Or as I call it, dialect, as opposed to Standard English. That is what I am grumbling about. It is fine when it is acknowledged that the speaker is talking in dialect, but in my view not good at all when used by teachers (who else in the first place would tell people to "stay sat"?!). Teachers are meant to speak Standard English to show their pupils the way, not so? I was wondering if US politicians and public figures also use this ugly expression. Frasier and Niles don't, and they know!

DAWood is right to talk about the terrible quality of English being used in Britain today. Errors involving number just for one: for example as you say, "the government are..."
The telephone will tell you "the caller did not leave their number" (whose number, who are these people with a number?). It is in every paragraph of every newspaper. The words "he, him, she and her" have all been replaced by "they" and "them". I believe the speakers' defence is that they do not wish to risk being seen as sexist. Instead, they prefer to remain incomprehensible. Even when the gender of the person is known. "The boy left their socks in the box". Whose socks? Who are the owners of these socks? Oh! the boy's! You mean "the boy left HIS socks ...".

Newsreaders, reporters, politicians. Written and spoken language. They don't care and tell you that you are a grumpy old pedant if you complain. Is it that bad in the USA? I doubt it. Frasier and Niles always get it right. or do they ...?

The occasional cock-up with "I" and "me", ... In England it is almost regarded as correct to say "Daphne invited Frasier and I to dinner". How about Frasier wasn't included: "Daphne invited I to dinner". I subject, me object.

It's bad, isn't it?

"How do you plead?" - "He pleaded not guilty". Correct English. "I sentence you to be taken ...and hanged by the neck ..."
Not pled, not hung. English law court language. Pictures are hung, convicted criminals hanged.
Your correspondent says you have to cry or weep while pleading. That must be in the US. In England you just wring your cap in your hands, and look shifty and apologetic. In 1970s South Africa you tried not to make jokes or seem frivolous, but it was hard.

In Scotland (and possibly England) it would be understood if someone were to say "well, her comment completely floored me" which I suppose means rendered me unconscious so that I fell senseless to the floor and, well, lay there. So "floorings" could mean occasions when such devastating comments are passed, as in "there was an amazing flooring at the Mothers' Union last night". But I think you mean carpets. Why not "carpetings"? Ah, if you are 'carpeted' by the boss it means you have to stand there on the carpet and be rebuked for your errors and/or failings. Boss to a colleague: "I did some great carpetings today". No better than floorings then.

"Floor furnishings" might do the job. Carpets, rugs, etc.

Yes AnWulf, but the man in the pet shop didn't know that because you were not there to tell him.

Euro English? I do not think there is such a thing. "He was sitting" imperfect active. "Stay sitting" imperative 'stay' + adjective "sitting" or "seated". "Sat" is either active perfect "the cat sat on the mat" or passive past participle: "He was sat upon by his boss" (figuratively, one hopes). To say "I was sat in the corner" begs the question "by whom"? - someone made me sit in the corner. But it is often used in local English dialect when it means "I was sitting". Walt ford says not in the US. Do any other nations encounter this departure from standard English? To this 'foreigner' living in England it grates on the ear.

And what is wrong with the good old South African greeting "Howzitt, you ous?!" which can be analysed as a contraction of "how is it, you, and 'ous' pronounced "oze", the plural of 'ou' which means 'old (friend)' in Afrikaans?

I guess the answer is that is is unintelligible except in South Africa where it is seen as warm and friendly. I guess too that elsewhere it may seem extremely scary, as it is invariably enunciated at top volume and always by someone who has just burst in.

mouse - mice. computer mouse - computer mice. goose - geese. mongoose - ?

Did you hear about the man who wanted two, and didn't know what to ask for at the pet shop? He thought about it, worked it out, went in and said "I want to buy a mongoose. And another one."

Yes, really!

Derek says that in the US the expression "the loo" is considered less refined and ladylike than "the shitter" or "the crapper"? What does this tell us about American ladies, or should I say 'dames'?

An American pupil of mine at school in the UK went to "the bathroom" every lesson, sometimes twice, and was considered the cleanest girl in the class. Didn't say much for my lessons, though.