I wonder if being in the bullpen is easier on a pitchers arm. Obviously the starter is going to pitch more innings but they do so in a very controlled way. They have a uniform amount of time between starts and off day throwing is very controlled and managed. Starting days have a well thought out routine that takes considerable time before the actual game. Within the game they get removed as soon as fatigue starts to effect the quality of their pitches.
Relievers pitch a varied schedule. They are asked to warm up in 5-10 minutes and then come into the game pitching with max velocity. Relievers don’t do the controlled side work that would help build skills and arm strength.
I don’t profess to know the answer and am unaware of any studies done on the subject but it might be an example of conventional wisdom that isnt really true.