OP's question is I think rhetorical, but this I believe is one potential answer:
“If you have to ask what jazz is, you'll never know.” ― Louis Armstrong.
Playing on a live stage through live amps is / was a thrilling experience for many performing artists. From what I read / hear, it is an entirely different experience from playing on an IEM stage.
So one answer might be that some musicians who are used to playing out with real instrument amplifiers have reported here on TB that the newer IEM systems feel like they isolate the performers (all band members, not just the bass player) from the audience, and from the sound of the instruments on stage and of the room itself. So even though the IEM mix may be technically perfect, to these musicians it reportedly seems sterile by compassion. This obviously does not take into account the problems encountered when band members don't know how to balance their individual volume levels. Or, don't want to. How many bands actually mike up their cabs now? There was a time when nobody miked up their cabs.
So here's something to put it into perspective. Monitors? We don't need no stinking monitors! Back in the day (I know, ancient history, but still...) we did three part harmonies with no monitors in front of Fender Dual Showman amps, Sunn's, Marshall's or Super Beatles sometimes cranked for everything they were worth. Yes, it absolutely can be done. So I'm probably not the person to ask. I really just wanted to say kids today have it too easy!