I'd guess that someone who jumps ship every year or two years will know more than someone of equivalent experience who's stayed at the same place for a decade. I learn the most the first six months at a new job.
Relatedly, there's the big benefit of much higher knowledge transfer of best practices in markets where job mobility is high. Individual companies, of course, end up giving up equal amounts of knowledge to peers and competitors, but ecosystems which support job mobility will outperform those that look down on it.
