I wonder why Windows Defender has the privilege to alter the system files. Read them for analysis? Sure! Reset (as in, call some windows API to have it replaced with the original), why not? But being able to write sounds like a bad idea.
However, I don't know what I'm talking about so take it with a grain of salt!
Some files under Windows are protected as the TrustedInstaller user, which is a more restrictive level of permissions than SYSTEM.
AV had traditionally run as SYSTEM on Windows (and, in the past, often had kernel mode drivers too). I've always thought it was a terrible idea. It opens up exciting new attack surfaces. Kaspersky and McAfee both had privilege escalation vulnerabilities that I can recall. There have been a ton in multiple products over the years.