Firstly, because it doesn't sound correct. Secondly, because it's just not correct. That's like asking why the past tense of the verb 'to hang' is 'hung', and why can't it be 'hanged'. I hang my coat on the coat rack everyday. I hung my coat on the coat rack yesterday. That's just the way the cookie crumbles I'm afraid. Thirdly, been is followed by the gerund form '-ing'.

"He has been sat at the table" = Somebody directed him to sit at the table. At some dinners or theatres, if you have a seat assigned to you before it starts, then either you will go where you are told or somebody will help you find where you are sitting. In either case, we would say that you have been sat at . "He was sat in the 3rd row" However, since it happens only once, you would never say "He has been sat at the table all day." That would mean that you keep getting up, walking around and somebody takes you back to that seat.

"be sat at" is, in my opinion, a correct expression. Although it may not pass to use it in an important exam. However, it does sound a bit local, and it also implies that somebody or something has made you sit at the table. Others: be stood at He's been stood outside for hours waiting for his date to arrive. He's been sat at his desk all day preparing for exams. Both, in my opinion, are correct English.

Well, for a start "at" and "on" are very different prepositions! The two examples you give create two very different images. The first example is not correct because it's simply confusing. Remember there's also a passive voice in English, and a reader can't tell if you intended a passive or active meaning. Also, it breaks the sentence and the end part sounds like a clumsy addition. Remember to stay conscious of the grammar you use. You just can't throw down a word, or a pattern of words, and expect it to make sense. Lastly (as already mentioned), "all day" suggests an extended action, so you need a continuous form: "has been sitting". "Sat" just doesn't make sense, I'm afraid.