The thing is, individual percussion instruments seem technically easy until you factor in the likely performance circumstances. The reason a guy gets paid the same to hit a triangle a couple of times to the other guy playing his violin non-stop for two hours is that the triangle player is also being trusted to count massively long rests accurately, stay alert during long periods of rest, and move around the stage. Not to mention that he also needs to play marimba, snare drum, toms, bells, possibly timps and piano as well in his job role.
Mastering the instrument is only half the work, and the learning curve of musicianship is equally hard for everybody. Any small child can bang a tambourine but how many can bang it with perfect rhythm and nuance for an entire piece?