Each baseball team has a certain amount of players on their team. There are some for each position. This also includes pitchers. Each pitcher has their own specialty pitch which they are known for. They may be known for throwing fastballs, curveballs or other types of pitches. Since the pitchers become trained in their certain pitches, they get used to it.
Just because a fastball pitcher is throwing the ball at a faster speed than the other pitchers, it doesn’t mean that they will get burned out quicker. Any pitcher can get burned out. I think that being burned has to do more with being tired of the game and not necessarily by the way he pitches. Burn out can be caused by a multitude of things.