Yes I see your point but it's a simplistic one, and not correct.
Grammar is nowhere near as simple and clear-cut as you make out because it is not, and has never been, planned, mapped out or organised. It has, and continues to, come about organically.
You're right that 'I was slept on the bed' is incorrect, but 'I was sat at the table' is not incorrect. It is, arguably, an example of non-standard usage (though not for much longer), but - and this is one of the most important things to remember about language, grammar and usage: non-standard does NOT mean incorrect, and what is deemed correct today was often non-standard, or even incorrect, in the past.
It is also used to impart certain nuances that are not there with 'sitting', particularly when the act of sitting has been enforced by another, or begrudgingly carried out.
'I was slept on the bed' obviously is incorrect, but as soon as we've heard it, it only takes a little bit of playing, asong, a catchphrase, and it could comfortably meme into a phrase.
Something I read in the 90s, an American syndicated columnist on speaking, words, was asked about the past participle of 'to bring'. 'Brought', they replied, 'brung'. Not 'brang'? Ha ha, nope, not 'brang'. Someone sent in some Neil Diamond lyrics, 'The song she sang to me, the song she brang to me.' Wrong,wrong,but a couple of months later the columnist found themselves using it in conversation..
I apologise/apologize for being unable to remember who it was, or where I read it, and I can't think how to google/ Google/ Duckduckgo it.
But our language develops accidentally, in my opinion, and we only account for rules later, by which time we've moved on.
Good thread.
