But good, bad &c are irregular so the short forms prevail. Also, you would say, "That is the kindest thing I've heard", but, "That is most kind of you".
We could argue this all day long. The general rule is that if an adjective consists of no more than two syllables you can generally ad er or est, unless it sounds ungainly. More than two syllables and you have to use more and most.
There may be exceptions but none spring to mind.
Cleverer OK. What annoys me is when someone says "I was sat there" or "He was just sat there". I feel it should be "I was sitting there" or "He was just sitting there". I am willing to stand corrected.
Interesting Maggie. Depends what is meant. If you mean was sitting it is wrong. However, you could say, "Mother sat her baby in his high chair", which would be correct. Depends if the verb to sit is transitive or intransitive. Oh, the beauties of the English language!