If you can't work out the chords to a simple-assed three-chord ditty like this, then what exactly do you get out of playing music?

This is not meant to be as rude as it might sound, I genuinely am interested in other people's perceptions.

An interesting question arises from this (higher up in this thread). I expect it might have been raised before though (?) so apologies if so.

I seem to find it easy to work out chords for most songs, although I admit I've had to struggle occasionally if the song uses 'odd' chords or has an unusual structure (I haven't made this very clear but I hope it vaguely makes sense). Likewise I can play fairly well by ear, and pick up tunes quickly, e.g. I can usually play along with a tune I haven't heard, unless again it's something pretty unusual. And I expect there must be people who can do it better than me, and woulodn't struggle where I do, perhaps because they have greater musical knowledge than me and can recognise every chord by hearing it (whereas I might have to work out if its a 13th or something, by trial and error).

Yet I know people who are trained musicians and can sight read in real time better than I can, but they seem to have trouble doing this type of thing, i.e. playing by ear, and would struggle to play along with a simple tune if they don't have the music in front of them.

Is playing by ear or being able to work out chords by listening to a song, something acquired by practice (perhaps because you are untrained and have to do it), or do you think it's something you either have or don't have? Maybe trained musicians don't have the need to play by ear and haven't developed the skill (if it is a skill).

What do others think? This is just something that interests me, no axe to grind or anything.