Maturity?

Market share, and the fact that Windows runs on a huge amount of diverse hardware and Microsoft does not want every hardware manufacturer to ship CDs with proprietary buggy drivers that destabilise the OS, any more than the Linux people do. (Perhaps Microsoft did encourage it at one time, but seem to have seen the light.) It's simply easier and more reliable when the "standard" driver is already in the OS and the device obeys the standard. Sometimes interests converge.