> In effect this means that while companies that give paltry pay bumps that don't keep up with the market may successfully hold on to lower performing employees, they'll be continually churning through top performers.

The same effect might even be partially responsible for the OP observation. I.e. the cause and effect might not be "job-hopping gets you paid more" but it might instead be "having a lot of skill gets you paid more and also enables you to job-hop more confidently".