The better question is why anyone would trust a third party to do operating system security than the experts who built the system?
Because I don't trust that the experts that built the system could stop everything. I like layered security because of this:
Safety is the first order, but things happen. When I got my new desktop MAC in early 2016 I wound up with Mackeeper and I have no idea how. I swear I did not download it. It took Malwarebytes to locate and remove all of the attending junk files from my system.
More than once I have gone to a site I use on a regular basis and all of the sudden a bunch of files will start downloading into my download file. These are not junk sites, but they were infected by hackers. Point - an ounce of prevention is a good thing.