There seems to be a stereotype held by some who can't read music that when you start looking at printed music your ears slam closed making you stone deaf, and simultaneously you lose all volition and ability to do different things with the instrument.
Some of those who can't play without written music seem to feel that if they just play some stuff, that upon the first clam or bum note they will experience spontaneous human combustion not even leaving a grease spot behind.
Neither of these exaggerations is true, of course, and the truth is that an enormous number of people can read music fluently, play by ear effectively, and do the various things between too. As I pointed out before, some things I and others do include:
Reading down the head of a tune from a lead sheet, then stepping away to play solos and backgrounds;
Playing while looking at written chord symbols;
Looking at a printed lead sheet but using ears to deviate from it in improvised fashion;
Looking at written directions of the sort "like Rhythm changes but 12 bar blues for the bridge";
Playing written music in an ensemble and following a conductor's directions on tempo, dynamics that are dramatically different than the printed music including jumps and cuts from one section to another...
All of these are a sort of "reading music" and none of them conforms to this stereotype of the unthinking trained seal reading musician that just pushed the buttons.
Of course, there are people who have vision issues, or who have issues in visual processing. They have to work out methods. that'll work for them. That's not what we're talking about here.