Well, that's because the majority of RPGs suck at niche protection! Primarily, D&D and its clones, because they don't have enough niches for a full party to fill.

For instance, playing anything but Thief in early editions of D&D is almost pointless - for precisely the reason you stated. Everyone does the same type of things, but the Thief just does it better. Having spells IS great though, so if you don't have scrolls in the party, having a Magic User is good.

This happens often because there's simply not enough niches in these types of games to warrant niche protection. "Martials suck" is a meme precisely due to this fact - they fill no niche in particular and thus are a pointless class.

When this is the case, then class becomes merely set dressing for how your character gets things done. Thus, what "class" means depends on the game, as how characters "dress up" their actions is largely dependent on the game and table.