Development Manager here. I will tell you exactly why. It is a LOT of work to find people. I'm not shy in the community either. I go to user groups and job fairs. I have always been the "You find people by pounding the pavement" sort of person, but it takes an immense amount of time. Between all my other responsibilities, it just becomes too difficult to manage at times. I try to get people without using them, because they are extremely expensive, but it probably frees up another 10 hours a week of my time that I can spend on other things that are as equally (or more) important.

This is on top of the fact that it is emotionally draining. Every person you talk to as a manager that you try and get to work for you is a sales pitch. To get people that you really want, you need to be engaged and energetic and positive about the workplace, even if you feel down. On top of this, reading resume after resume is actually draining. There are many, many that are poorly written, and you have to slog through them, because 1. you want to see if that person really is any good, 2. sometimes you don't have a large enough pool that you can just immediately discount someone. Having someone clear out the first round of all of that saves quite a bit energy.