Because its based on distrusting yourself and in general the user. (Edit, thats not nesisarly a bad thing.) However AV software goes way beond a simple "sanity check" these days. AV is for corperations to distrust employees now. These strong arm distrust fetures are "acceptable" on company property but walk all over my boundries for a personal machine. Use whatever is built into your OS (and NOT your OEM. Like from Windows etc... and not from Lenovo etc...). if there is none built in, it means its your choice if its needed.

Edit: if I failed to explain my point or if its bad, explain why.