You also have to consider that many pitchers are "fighting for their life".
There are studies suggesting that "sitting" farther from your max (I think the threshold was like 4 mph or more) helps your health while guys who sit within like 2-3 mph of their season max for most of their pitches would get hurt more.
Justin Verlander actually did that a lot, he could throw 99 but he would often start the game like 93-94 and only reach back for the 97, 98, 99 a couple times a game when there is a tight spot.
That worked well for Verlander but if you are a guy sitting 92 and maxing 94 and you are just clinging on to a sub 5 era you can't afford to dial down his sitting velo to 90 or you get send to the minors or DFAed.