Because "playing on the guitar" (or any instrument, for that matter) is incorrect. The right way to say it is "play the (instrument)".
That's unless you want to say something like "My parents like to play -this song- on the guitar"
I suppose that the problem is not the position of "sometimes", but that it can't go together with "are playing". "are playing" means they are doing that just in this moment; sometimes thay PLAY guitar (repetitive action, not necessarily now)