Reasons:
a) They can buy cheaper components to build same model.
b) They can produce new models cheaper.

(battery? stock battery life is 5h, even less, under moderate stress. If you work more than 4h59min/day, you need two-three bats or a charger int your pocket... so no matter if OC under heavy stress longs 2h30min, the 5800 Xpress Charger does the job... as with stock kernels)