Salary isn't the only thing that people are looking for. Especially if you're early in your career, your interests and abilities aren't clear. As you learn more, you find you enjoy one thing more than another, are better at some things and not at others. Also, early in your career, you have fewer options, so you may start out in an area that's not close to your specific interests.

I'd guess it's more likely than not that someone who's learning lots and quickly will find that they're better suited, and thus more valuable, to a different employer than their first employer. It's not simply a matter of raising wages. It's getting a close fit of aptitude and interest with the work at hand.