So are they leaving because of pay or culture?
It's both. First of all these jobs are not all that well paid as someone might think, and with OSV it's even worse, because instead of money you get "honor to work for/with the best" so salaries can be worse. Expectations and all the shit you get when things go wrong are way higher then anywhere else in the world, which is not really big plus either. When things go right, servicemen are never noticed anyway.
But there's another thing which makes this transfers more difficult. Once you change job, you basically make it very vey hard to find new one, as in this business noone wants people who are changing jobs. It's not really rocket science thing, but many take it like it is, and with serviceman changing job, he takes all those rocket science secrets to competition. When he did it once, he will do it second time too, so companies (or teams) don't really like such guys.