Way back in the day, everyone reported at the same time. But historically speaking, they have reported first because it takes pitchers longer to get to a point where they can even throw 25 pitches. They also have some specialized drills that the rest of the team does not need to participate in (covering first base, backing up throws, blocking pitches, pick off attempts). Catchers have to be present early because they are integral into getting a “feel” for members of the staff. Kelly Stinnet, for instance has to familiarize himself with Pavano, Wang, Chacon and others.

Also, the modern tendency is to bring the pitchers along slowly. First the sof toss, then the long toss, then fastballs only, etc., etc…..

There is a split among baseball people whether to use pitch counts or whether throwing more helps or hinders. The most vocal proponent for pitchers throwing everyday is Jim Kaat. Kaat believes the more a pitcher throws the stronger his arm gets. This carries an important distinction though: fastballs only. If pitchers throw all kinds of pitches that is what causes stress on the arm and increases the risk of injury. But old time pitchers like Jim Kaat believe the more fastballs you throw, the stronger your arm gets.

The pitch count phenomenon is really a function of guaranteed contracts. Who wants to risk hurting their investment?