Its because as Laika said earlier, some guys are throwing max effort and throwing 93. Velocity is not the factor, its whether they are throwing max effort or not. It's like youre assuming that pitchers who arent throwing 100mph are not throwing max effort, when the reality is that in todays game, the majority of pitchers are throwing max effort, they just cant all get to 100mph with said max effort.
And yes before you pick that apprt, of course there are other pitchers that do not throw max effort. There's no absolutes, just trends.
As for the talent pool thing, it is deeper. It's a fact of life in every sport that as the sport moves forward, the talent pool gets bigger, and the replacement level player gets better. The gap between the best and the worst gets smaller and smaller incrementally over time.