Differing goals. For realism, you're right. Many systems forego a critical failure entirely because of that.

But fumbles are fun and dramatic. So while 1 out of 20 is excessive, having that danger lurking in every roll can be exciting. Of course you don't want to chop off limbs at every fumble, but chucking a weapon, breaking a bow string, insulting an official... They move the narrative forward in interesting ways.