Because it sells the hardware, where they make the most money. I don't think they will do something like that, but I think the reason is that they just don't care enough to dedicate internal resources.
Well, even for BootCamp, a marketed feature, they only dropped a halfarsed set of drivers which worked very poorly and were pretty never updated. We need to use hacked AMD drivers to get updates for the Radeons and the Macs still overheat and burn a lot energy due to lack of even basic power saving features. Heck, even the GPU switching isn't implemented.
Why would there be expectation they'd do anything more for Linux?
Windows in the Bootcamp role was also compliment ary, in that it enabled a company or individual worker to consider Ape hardware where Windows was a requirement or desired as a personal preference.
This also enabled those users to spend time in MacOS or switch to virtualizing Windows and may lead to one or more personal purchases for themselves or members of their household.
They don’t run unsigned binaries, but they run self-signed binaries (to the same extent that Intel Macs run unsigned binaries) and the linker automatically does the signing. It’s an architectural simplification, not a substantive tightening of the screws.