My main reason for using Microsoft Defender is the business model. It's in best interest of A/V companies for people to have viruses, it's in best interest of Microsoft for people not to have viruses on Windows.
The difference is that Microsoft has more incentive to optimise their AV against both false positives, and false negatives. And it can afford to stay invisible if there is no threat. A commercial AV has to make its presence known, and most I've seen do this constantly: If you never get a virus and the program remains silent all the time, people will wonder if they really need a commercial AV. If you never experience trouble and Microsoft Defender stays silent, you have a happy user: Windows works without issues.
Also, top AV are better at catching viruses and have less performance impact than Defender.
https://www.av-comparatives.org/tests/real-world-protection-...
https://www.av-comparatives.org/tests/performance-test-april... (Recent Defender has the most impact on system performance on all AV tested)
Obviously, it's up to you to choose between:
* Using Defender and suffer the worse system performance impact of all AV
* Not using AV but a higher risk of catching viruses
* Use third party AV with better detection and less performance impact but risk opening new vulnerability on your system.