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...