JGZ re own vs generic chipsets
Developing drivers for generic chipsets saves all of that time and energy re-inventing the (hardware) wheel. Companies developing chipsets, graphics cards, etc generally know what they are doing and spend a lot of time optimising their products (video cards for instance).
If your OS supports many chipsets then as a hardware manufacturer you can tailor your OEM hardware product to the wishes of the customer. Neither you nor your OEM customer are dependent on just one chipset or expansion card supplier.
If you buy a generic chipset and the supplier accidently includes/reinvents something patented by someone else, they get sued, and the worst that happens is you have to find a compatible chipset.
If you develop your own chipset and you accidently include/reinvent something patented by someone else, you can be sued and there is no redress.