Companies have an incentive to make their employees more productive, but other things being equal, you'd prefer to teach your employees non-transferable skills rather than transferable ones, since non-transferable skills can't be used on the job market to bid up wages.

There's a comment here about how conferences are great to help your developers make connections in industry. That seems like a mixed bag -- why should companies help their best developers to find their next job somewhere else?

There's an interesting Econtalk episode on the book "Learning by Doing" where this came up:

http://www.econtalk.org/archives/2016/05/james_bessen_on.htm...

[To summarize: when the textile mill was invented, skilled individual labor became much more valuable (because skilled workers were much more productive), but wages didn't rise much over a period of 50 years. This finally changed with the introduction of interchangeable parts in machines, which made job skills transferable, which allowed skilled employees to demand higher wages from their employers.]