They allow a 3rd party to change Kernel stuff with an update. Apple banned this a while ago.
Antivirus software always works as a driver in the kernel, no other way. You'll get the same in Linux, for example. In MacOS it may be slightly better (if I remember right Darwin is a micro-kernel), but in fact a broken driver still can crash the system there.
You're confidently wrong: https://developer.apple.com/support/kernel-extensions/
Made it a lot harder for everyone involved, but still possible, as it’s a very useful technique.
PS: Since I'm being downvoted, here is the link showing that it is still possible using Reduced Security:
https://support.apple.com/en-gb/guide/mac-help/mchl768f7291/...
I doubt Apple will completely disable kext in the near future. Making it hard enough to be impractical has most of the benefits already.