In any poker hand that isn't the nuts, the most important thing is not the absolute strength of the hand, but the relative strength compared to what other hands people might have.

In the full house hands you mention, in absolute terms you have a full house. Woohoo! But if you bet with your underboat, and get raised, then there is a very strong likelihood that you are beat, firstly because there is a very obvious and likely hand that beats you (Kx in your first example) and secondly because the board is so obviously dangerous that a worse hand - say AA - wouldn't risk a raise here.

One reason we beat donks is because they only see the strength of their own hand. A2 on a AA6J3 flop against a raise? All in because OMG THREE ACES, despite the fact that ANY other ace beats you, as does any set. Indeed A2 is the WORST hand you could have here against any strength because worse hands will get the hell out. I would find trip aces with A2 here pretty much as easy to fold as Ace high on an unpaired board, but donks don't, and your example is effectively looking at the same concept.