Mostly because Apple is so rich that they don't care. If they were not swimming in a pool of money this wouldn't happen.
Interestingly enough, Apple helped to develop a version of Linux running on the Mach microkernel, and handed out thousands of MkLinux CDs at WWDC and MacWorld Boston in 1996. Macs have been running Linux in various ways ever since.
https://en.wikipedia.org/wiki/MkLinux
http://gate.crashing.org/doc/ppc/doc003.htm
Windows has also been running on intel Macs since Boot Camp in 2007. It remains to be seen whether ARM Windows will ever run natively on Apple Silicon however.