Because you have people watching Smallville, Batman: The Brave and the Bold, The Dark Knight at the same time.
Because there are people who will play Batman: Arkham Asylum and Batman: the Brave and the Bold.
Do people get confused by all this? NO.
So, this is why I think less continuity could end up being a good thing - you pick and choose what matters to you. So say, Superman: Earth One isn't your cup of tea (it certainly isn't for me) - that's fine; there's still every other version of the origin and the coming of age out there. So pick what you like, support it with your dollars, and that becomes the only "canon" that matters.