I've experienced this over and over with multiple performers: experienced and novices, instrumentalists and vocalists, groups and individuals, it doesn't seem specific to any group. They all think their performance is "good enough" while we're tracking, and then want to spend a lot of time in the mix trying to fix and improve things that really should have fixed in the original performance.
I have a small hobby studio and work with a lot of amateur/hobbyist performers, so often I end up fulfilling both the producer and engineer roles.
How do you as an engineer or producer convince your performers that it's better to spend the time to "get it right" while tracking as opposed to fixing everything in the mix?