Generally Apple is only expected to make their own touchpads work well. So it's fewer devices to develop/test, the OS folks can talk to the hardware devs, see their designs and firware and even get to influence hardware decisions. Perhaps Apple puts in work for a handful of the top touchpad brands, but they also are incentivised to work with Apple.

Compare to linux, where they have zero influence over any of the hardware involved, and are expected to support every single hardware combo possible.