It's also that humans are very bad at repetitive detailed tasks. Sitting down with a code base and looking at each function for integer overflow comparison bugs gets boring really fast. It's a rare person who can do that for as long as it takes to find a bug that they don't already have some clues about.

It's the flaw in the "given enough eyeballs, all bugs are shallow" argument. Because eyeballs grow tired of looking at endless lines of code.

Machines on the other hand are excellent at this. They don't get bored, they just keep doing what they are told to do with no drop-off in attention or focus.