Yes, it must take a big ego to be a successful MLB player. Yes, time is cruel and players think they can defeat it for far longer than they can. Like Castellanos, Kepler apparently lives in a fantasy world. He, in effect, has been the starting LF. I doubt he was promised that he would start every game. I checked his career stats to see if this sense of having to play every day was in any way justified. Answer: no, it isn't. At the time he shot his mouth off, he was on a path to having the most ABs he's had since 2019. He was a 4 rWAR player that year, this year he's on a pace for 0.2. That year he had an OPS+ of 127 against LHP. This year it's 79. Of course, back then he was 26 not 32. He complained when he had 'only' 273 PA, but last year it was only 399 for the whole year. He is being overpaid to play a TEAM sport and is not close to holding up his end of the bargain. You don't play well, you play less.
Castellanos is a negative WAR player for us, earns $20 mill, and somehow decided it was 'right' to have a visible tiff with his manager for being taken out, late in the game, for a defensive replacement. Everyone knows he is a lousy defender in the OF, but that apparently escaped his notice. Luzinski accepted being replaced in games for defensive purposes. He was hitting a heck of a lot better at that time
I think a fair share of the blame does fall on the manager. He has babied his veteran players and created a huge sense of entitlement. What is needed is an explanation of how the game is played today. Teams have metrics on everything and projections of what is most likely to yield a positive outcome in a given situation. Even the Phillies don't go by the career numbers on the back of your baseball card now.
The Phillies have apparently become a team of selfish, entitled, downside of career position players. That's not winning baseball.