I think you're missing the cost/benefit calculation. Using any software makes you vulnerable to your software being compromised. But we take that risk because we get something out of using software. If you want to run Windows apps, you need Windows. AV software is different because they open up vulnerabilities, but don't give you any benefits. You can't run Excel without Windows; you can run it without AV software. So adding AV software requires some careful thought as to whether or not the cost/benefit is worth it. What the above comments are saying is that in a world where Windows Defender exists, installing third-party AV software is not worth it. The cost outweighs the benefit.
Defender doesn't get updated as fast as other AV that is why it is free and if you actually get a MS enterprise AV is a ton of money compared to other enterprise AVs. I have had Defender not find viruses other AV will caught as long as a week. Also defender also opens up vulnerabilities. I didn't miss the cost/benefit calculation.
https://www.cvedetails.com/vulnerability-list/vendor_id-26/p...