contrary to what you may believe, some people need to work on content and should not be touching security. it's absurd to expect apple to fire all their non-security related staff and replace them with a whole organization of engineers that only work on basic security.
I do agree that apple has had a ton of problems in this area and needs to work on it, but this example is very played out and boring.
You can't just hire arbitrary numbers of security engineers. These positions are difficult to fill. There's a good chance that Tim Cook wanted to hire more security engineers than was possible.