It's interesting that people/players only complain about starting pitchers being removed. Why don't hitters complain when they get pulled for their platoon counterpart? Why don't relievers complain when they get pulled (especially before the 3 batter rule came into effect)? Isn't Zeigler pissed when he's pulled? Say it's 2019 and Zeigler strikes out the first to batters (both of whom are right handed) and there's a lefty coming up (which means statistically, he has a much lower chance of success) - doesn't Zeigler think he's pitching well and can get the next batter out?
I suspect the answer is simply because it's more common and players are used to it. Players seemingly understand and accept the reasoning behind it more (ie, Zeigler knows he's statistically shittier vs. left handed batters and accepts why the manager is pulling him - even if he's dealing and struck out the first 2 batters). We just aren't there yet with starting pitchers - it will come with education and time. I have to imagine there were transitions when relievers started to be a thing - and then when 1 inning relievers transitioned into the game (instead of the multi inning reliever).
Glory - I think we understand that players hate getting removed from the game. Their competitive nature is a huge component of their success. But there are lot of roles in baseball (and other sports) where players willingly come out of the game (even if they are playing well).
That said, I am surprised hitters these days aren't losing their s*** because the number of platoon players seems to have skyrocketed over the past 5-10 years without huge complaints about not being full time starters, playing every day and getting paid.