On what basis are bikes categorised?
Weight and durability are indeed factors, but it's also about the performance of components. The "high level" bikes are for a "high level" of performance. They're usually designed with competition in mind. 

The shifting has to be perfect, the brakes need just the right amount of power, the suspension has all kinds of adjustments, tires specific to the terrain on that day are used, etc.

Of course, Average Wealthy Joe will buy one of those bikes and brag about it, but hey, be happy he's financing your favourite bike companies. ;)