Not just the risk of them jumping ship, the problem is that because mid to senior dev salaries are highly desirable it attracts juniors into the field who are n

Not just the risk of them jumping ship, the problem is that because mid to senior dev salaries are highly desirable it attracts juniors into the field who are not suited to the job but like the idea of working in the industry or the potential money in it. So, you spend time training a group of people where some aren't suited for the job but you spend a lot of time on them and the better ones leave quickly. If you don't pick the juniors carefully in the first place and if you don't have the environment and renumeration to keep the better ones from leaving then it's probably better to just not spend too much time on recruiting them. It actually might work well for a company that doesn't want or need a large dev team. Also, not everyone is good at mentoring, some of the best people are often self taught and some of them find it difficult to teach others.