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.
Biggest parlay odds winner… feels like that’s way harder to pull off compared to slots. In slots, anyone lucky enough can hit a x10,000 multiplier, but in sports betting, that kind of return seems almost impossible. I wonder why there aren’t many contests with this format. If there was one, I’d be happy to join. It’s not even that costly since parlay results take time and aren’t instant like slots.
There were some that you missed and maybe you are not playing on that casino, that's why you missed it. One of them is in Shuffle, what you need to be the first to beat the highest multiplier from the previous winner on parlay bet to win $2500. It was running for more than half a year if I remember it correctly. The other one was in betcoin with more winners but lower prizes. I believe there are some other places with similar contest, so you need to explore more sports betting sites if you really like to participate in multi-bet contest.