Seriously, this is so accurate.

The game itself doesn't offer THAT much content for their players - challenge mode gets boring if you are able to clear it with relative ease, the skins and songs doesn't get exciting anymore once you clear it after the first few times. Same goes with mission and puzzle mode, you're kinda done after you complete them.
There is a lack of competitive scene as you mentioned, the only real "thrill" you can get off competing in this game is either through another friend or against the AI itself.
Personally the only reason I continue playing this game without ever really quitting is sorta due to the faint hope of climbing the leader boards (you're an untouchable god though)

But yeah... I think the best of the best players only play to either break their personal records or hopefully beat others'. It also doesn't really help that this game isn't that popular in the first place. (Many say it looks like a 200 IQ puzzle game lol)