For me, it's because wagering contests are not meant to reward players but to make users lose more specially if the wager contest only counts on games with house edge.
I usually only win a wagering contest when the online casino has just launched and there are only a few users playing, so the chances of winning are higher. Also, they don’t count sports wagers, only like slot wagers because the idea is for you to lose more and keep chasing your losses.
I also have the same winning experience when it comes to wagering contests. Once there's actual competition and the big users start to flock in, i'd rather give up and play normally. It's very rare to see contests with little competition, but it still happens whenever several casinos launch around the same time and one of them overshadows the rest.
Wagering contests for sports are just not that effective, and it's why bookies prefer to push for parlay or multiplier contests instead, because gamblers tend to lose a lot with those kinds of bets.