As I understand it; Windows Defender is written and monitored by the Windows people, who know the operating system and code, and it is in their interests that machines running Windows do not fall foul of viruses.
So Defender is regularly updated and available to update Windows registered machines as part of the automatic Windows update, as each new attack is detected and neutralised. I would imagine that Windows have many more programmers than an aftermarket product?
I was told that Defender is much better than aftermarket virus software, but apparently they are not allowed to advertise the fact owing to competition laws?
PS, if aftermarket software is free, then what's in it for the producers of that software - what do they get out of having their free software on your machine?