The most popular desktop OSes have decades of pre-existing software and APIs to support and, like a lot of old software, the debt of choices made a long time ago that are now hard/expensive to put right.
The major desktop OSes are to some degree moving in this direction now (note the ever increasing presence of security prompts when opening "things" on macOS etc etc), but absent a clean sheet approach abandoning all previous third party software like the mobile OSes got, this arguably can't happen easily over night.