I get you but here are some factors that may contribute to that whole thing being customary. 1) There are usually more people in the audience than there are on stage. So while its less mixes its more ears to please. 2) The people on stage are getting paid to be there. The people out front paid to be there. 3) Before there were IEMs we all mixed on these things called wedges and side fills. It was a great way to cut your teeth learning your frequencies as well as the character and limitations of some of your gear in terms of loudness and feedback. Im probably gonna catch some shit for this but mixing ears is fucking easy. Like literally keep your eyes on stage and take care of your people. If youre on tour mixing the same songs every night, set up scene changes mix post-fader if you want. Theres obviously plenty more nuance Im not addressing but thats what Ive got for you.