My guess is that they just don't care to spend the time on it when the majority of people wouldn't even notice. But of course those of us that would notice would really appreciate it.

As a guitar player, it equally irritates me when the person "playing" the guitar has clearly never touched a guitar in their life. Similarly, when an actor is actually playing it, I really appreciate it.

Off topic, but I once walked in while my wife was watching some anime where the guitars were all extremely accurate, like down to what tuning pegs they would have had for the era the guitars were from. They must have motion captured all of the guitar playing from when they recorded the music, or at least took video of their hands, because they animated it perfectly. Down to the tapping parts and everything. It was jaw dropping. I made her start the whole show over from episode one so I could watch it.