Because people like to give names to things so they can talk about them more easily instead of constantly having to describe them.
It would be a bit silly to not make categories in bikes and simply call all of them "bike", wouldn't it?
There is a huge difference between road racer bikes, full suspension downhill mountainbikes and beach cruisers ... if you do not want to make that distinction and you insist on doing downhill races with big jumps on your full carbon road racer, be my guest
Or if you really want to go road racing with a beachcruiser ... well ... to each his own, right?
Going beachcruisin' with a full suspension mountainbike ... you'll be fine (because anything beats a cruiser), but it would be overkill.

Face it ... there is a need to label bikes.
Not everything is easily labeled though ... so a bit of explanation is usualy still necessary.