Maybe developers stop learning because they realize that that to learn how to do the same thing with 167 technologies is not a real development. Maybe they realize (they should) that self-development should be we well balanced, so after programming workday they should invest their time into sports and work with their emotions and take care of interpersonal aspect of their lives.
The older I am the more I'm convinced that development is not individual thing - it's a team sport.
Hey, company: divide your workday into 4h work for external projects and 4h work for team development.
Smart, mature developer, will realize that the most inhibiting phenomenon for him is usually the job itself.