I think it's because they all want to be everything (virus scanner, firewall, email scanner, web scanner, etc) that they bloat your system doing all those functions. I use windows defender because it seems to not bog down my system as much.
I have seen a few ads already this morning of the pay-a-license-fee-each-year software that's advertised all the time on radio and tv. I've heard it gives the illusion of working to protect your PC by deleting important OS and installed software files so it ends up harming rather than helping your system like proper software should.