* The money to pay a junior developer who won't be productive until they're trained (probably a few months).
* The money to pay a more experienced developer to train someone rather than working on your app.
* The wiggle room on your deadlines to be able to slow things down for training.
* The stomach to live with the fact that an experienced developer may become available two weeks after you've hired a junior dev.
A bootstrapped startup is almost never going to have the capital to make this feasible, but a big company might.
Java developers are easy enough to find, so it rarely makes sense to spend months training someone. Ruby developers are a bit harder, and the training is almost mandatory if you're looking for Haskell developers.
That said, I would like to see big companies take a leadership role in this stuff. My company is always complaining about how hard it is to find qualified people. We've got the time and resources to run a bootcamp-style training course. We could bring in 30 people and train them for a few weeks and offer junior positions to the people who performed best. The people who were passed over still have valuable skills they can use to be hired by other companies.