It's one thing to catch in a game, sit back there for 15 to 30 pitches, and then get a half inning off until you have to do it again. It's another thing to sit back there for an hour straight just catching pitches with no break. Watch his knees.
Also, for whatever reason, some pitchers are as wild AF in the bullpens. And, nobody cares that the catcher is getting the wrong end of it.
Lastly, right or wrong, in college, the older classmen catchers always seem to come up with minor excuses to not catch pens. I'm sure there's some of that in HS too. Be ready for it, although there's little you can do about it.