I think from an evolutionary perspective most of us aren't wired for long focused mental work, especially not after studying/working the whole day already (that's why most side projects fail). It had very little evolutionary gain throughout history. I think life was mostly about surviving physically and then socializing and chatting/games in rest time. I'd note that even when we lived in caves we had to do mental work (like building a shelter) but it was never for months or years end and it was always very physical. The physical part makes it much easier to persevere for biological reasons (adrenaline, edrophins etc), and also our survival was at stake (!). Compare that to learning to play the violin which has no significance to survival at all.
So it makes sense to me many of us have an inner resistance when we want to exercise/read a hard book/learn to play the piano even though intellectually we prefer those activities. The people who still manage to do that and persevere somehow have a subconscious that values those activities highly. It's the best explanation I can come up with - it is what it is because of how we are wired and because fast gratification activities made a lot of sense when we lived in caves.