always seemed to me more like work than fun
NFL is definitely fun, I think that is something that will always remain popular if the legal picture stays the same. It just lends itself to a lot of fan engagement, the player pool for NFL is so big you can get in tons of different games, etc
NBA is pretty much work though. Similar to the time needed to put into trading stocks or options. Player role/injury fluidity really changes fast since there are games every night, I don't know why anyone would find this fun. I enjoyed it at first but now I pretty much just do it because it's a hard way to make easy money. I'm a big NBA fan but there is nothing fun about being at your computer at 6:45 waiting to see of Bradley Beal and DeMarre Carroll are out so you can deploy their backups.
MLB I can't comment on too much, I don't follow baseball at all or find the sport entertaining. I do know it lends itself well to DFS because you can stack lineups in tournaments pretty easily and lineups are released hours in advance so you don't need to keep up to the minute injury info. Just isn't my thing and I'm not going to play something just for the hell of it cause I'd just lose.
Golf is actually huge in DFS. But I don't play that either. A lot of people seem to like it though.
Fun is very subjective. Some people find blackjack, slots, the lottery, scratch tickets or craps fun, I don't know why, but they do.