>people not understanding that [arbitrary mildly repetitive task A] might be a source of entertainment for someone
>the same people enjoying [arbitrary mildly repetitive task B]
Does your dad like to solve crossword puzzles, or he does enjoy cooking or working in the garden? Hell, most of the things people ordinarily enjoy are at a similar level of complexity as osu, or probably even below that. It's staggering how people refuse to acknowledge how their own hobbies tend to be just as basic as anyone else's hobbies. If you think about it, simplicity is an essential for most people in the first place. When things get too complex/mentally taxing, they stop feeling like a hobby and start feeling like work. How many people solve complex mathematic equations for fun?
Also, most hobbies probably look boring when you lack every sort of knowledge regarding the general field the hobby is located in, since you can reduce them to something very simple most of the time.
So why wouldn't you, or anyone for that matter, enjoy clicking circles for fun? Your dad needs to wise up.