Basically because people will pay it.
The industry thrives on creating new "game changing" standards which a lot of people will quite happily throw most of their money at, thinking they will get faster, or that they need it.
Trouble with this is ,once a bike is 3 or so yrs old, its considered to be behind the times and depreciates quick.
If you can suffer that, go for the boutique stuff, but a $4000 runout from one of the big brands will be just as fast/much fun, and will last.
A ktm350 exc is under 13000 . Is any bicycle is worth that?