For the same reason we don't reinvent the wheel. Or perhaps, the same reason we don't constantly change things like a vehicle. It works well, and introducing something new means a learning curve that 99% of folks won't want to deal with, so at that point, you are designing something new for the other 1% of folks willing to tackle it. Unless it's an amazing concept, it won't take off.

> Or perhaps, the same reason we don't constantly change things like a vehicle.

Are we stuck with the same brake pedal UX forever?