I don't have the reference at hand but it was part of their various anti-trust fallout, as it would give them an unfair advantage regarding to their own products.
PS: an analysis of that situation during the Crowdstrike issue, with the relevant bits of the EU ruling: https://www.computerweekly.com/news/366598838/Why-is-CrowdSt...
They could offer an API for security relevant scanning for EVERYONE, including their own antivirus software. But that would make the world better, and make the legislation look justified.
It's the exact same thing with the Google Maps integration on Google Search. They could offer an API and a selection of map provider to the user. That would make Google Search better for the user AND enable competition. Instead they threw a temper tantrum and disabled map integration entirely, so they can blame the EU.
APIs means Microsoft gets to dictate what products exist in the first place. We know security software is a use case, but if for instance VR vendors come up with a completely different use case, will the existing APIs be enough for them ? And what recourse do they have if Microsoft either doesn't give a damn, or purposefully drags their feet for whatever reason ?
I mean, they have _lots_ of APIs. You'll just never have as much control from usermode as you will from the kernel; what API would make you say "yep, don't need a driver now, can do security fine from user"?
Microsoft claims they need kernel level access to implement their paid for security product ( Windows defender while free for home use is not free for enterprise )
If they firmly believe that, then they cannot block other software from having the same access or it would be anti trust.
They could perhaps make it free and bundled in windows, but they don’t want to lose the enterprise funding.