It was full of ad infested solutions, which would crash your computer from time to time.
Defender at least was reasonably performant and tended to be stable.
You could say that since they had access to kernel source, they were better informed, but I guess if there was an API, the provided documentation would solve the issue (not necessarily, not everyone bothers to read the docs).
But then you get back on how to enforce equal and open access for everyone (the EU did try to make Microsoft open the Word file format, but turned out it was so complicated and documented in legacy code only, that Micorsoft had trouble giving useful docs)
Anyway, as you said, it's complicated...