Why do the vast majority of these seem to either block off the 40pin header or convert it to female header that's incompatible with further hats? And why are they full sized, despite having minimal circuitry on them? I can appreciate the ones that build in a cooling system, but that doesn't need to block off all the pins that aren't being used by the hat.

It seems like all this could be accomplished with a small board that doesn't interfere with everything else one intends to do with a Pi or Pi-clone. In fact, I'm surprised at the lack of built-in POE Pi boards out there.