What a lot of thoughts!
As someone who started in classical music, I learned to sing or play anything put in front of me, once through, and then perform it. But never having to memorise it.
Take the book away and all was forgotten.
Coming into folk music later in life, having to make out what notes others were playing (and they didn't all play the same notes), and then remember them, was tremendously difficult at first, and still can be a bit of a struggle. Being able to find the dots in books or on the net was a great help, but committing to memory very difficult.
Others can remember things heard once, but struggle to sing/play from music.
Surely there's room for all of us?
Re ossonflag's comment about actors, many radio shows were produced at short notice and the actors saw their parts for the first time as the show was broadcast. To be able to bring this off convincingly is a trade like any other.
The other big thing is, if we're making music in private then mistakes can be overlooked, but frequently we're in a pub (not many church halls with a bar!) so there are members of the (not very interested) public there. If our singing/playing is so bad or so repetitive, week after week, that the public cease to come in, then we will not be welcome in that establishment. Yes, we are there for our enjoyment, but that shouldn't be at the expense of theirs.