Because i think it is a better and more interesting game when some characters will be bad at certain things regardless of build. It is obviously not for you. But personally i like the idea of some classes being bad at aspects of the game and this being part of how they are balanced overall. I just find the whole approach where everyone is basically good at combat dull. Same if everyone is basically good at exploration or basically good at social interaction or investigations.
Yeah... I just don't get it. To have a class be deliberately bad at something with no opportunity of getting mediocre (let alone good) is pretty much having Roles by another name. Based upon your comments on the earlier pages... it seemed like you were someone who didn't want roles hardwired onto classes. But apparently you do. And you want those roles being MUCH WORSE than what the game currently has.
I suspect you are in an exceedingly small minority on this one. And I don't think you'll ever be able to find a class-based game which will satisfy you, because no designer worth his salt will ever design a class to be THAT BAD at something. It makes no sense. There's no reason why the player can't just do it, rather than the game forcing them to.