In other words, once the employee has the skills to earn $85k, why should she wait six months for her giant raise when she could just switch employers and get the giant raise right away?

Not that this is the reason why the fantasy scenario doesn't work - if it were reliably possible to turn a raw recruit into DHH with six months of training, the fact that we'd have to pay market rates the entire time would be the least of our problems. Indeed, it would not be a problem at all.

A real reason why more companies don't try training up raw recruits is that (a) running a school is a specialized business and (b) the yield is far less than 100%, so it only works at scale. You need to admit - optimistically! - 10 or 20 bright and motivated nonprogrammers in order to graduate one person who, after six to twelve months of training, can be expected to successfully attack problems like:

"A customer has called; he has an obscure problem with his web site. Help him debug this problem over the phone. You have no access to the code or the server."

or

"Here's a legacy codebase that spans 267 files, two major versions of Rails, and three generations of programmers. Improve it. Don't break it, though, because our revenue depends on it."

or

"Here's a collection of 175 cloud instances running in Amazon. Build a system that reliably backs them all up once a day, with no downtime, and that can verify on demand that those backups exist."

or even something as "simple" as

"Here is a Wordpress site with a handful of specialty plugins installed. Here is an empty Git repository. Fill this Git repository with Rails code that implements a site that looks and acts exactly like the Wordpress site."

[1] http://www.azlyrics.com/lyrics/andrewbird/howyougonnakeepemd... - this song is now 95 years old, so I guess I better footnote it!