If there's enough money to put on both sides of the outcome, Las Vegas will handles the bet (and take their cut): That's how parimutuel betting works.
I suspect, there's just not enough folks who want to jump in the pool.
Sports betting is not parimutuel betting. Sports betting is mutual betting. There is a BIG difference.
Parimutuel betting (mostly horse racing) means your bet is tied to every other bet in the pool. For instance, let's say you bet the Kentucky Derby on the 5 horse to win. Your bet goes into a pool with everyone else that made a win bet, no matter where they bet or how much they bet, whether it was at the track or at a casino. The odds are "live" on the bet until the race starts. Let's say the 5 horse is 5-1 when betting opens, and you bet it then. That does not mean you automatically get 5-1 on the horse. It could get bet down or up by post time, and those are the odds you will get. The takeout/juice/vig on the bet is dictated by the state racing commission (it is usually 15% on a win bet) and then they have to payout all of the winning tickets out of the win bet parimutuel pool ONLY. That is why you will see weird numbers pop up on the tote board, say your horse was 2-1, that should pay $6 for a $2 bet ($4 for 2-1 on $2 bet plus your original wager)... but you will see the win pay back $5.20 or some such.
Mutual betting, like sports betting at a casino, is a bet between only you and the house. It is not tied to any other wagers or pools. The odds are "dead" on your bet with the house, meaning if you bet State -7, you get State -7 even if the point spread changes by kickoff. The hold/cut/juice/vig on a sports bet is usually 10%. They only take the cut out of the winning portion of your bet, so a winning $100 bet on State pays back $190.
You are correct that the lines are set on sports/mutual betting to attract equal action on both sides, that guarantees the house wins no matter who wins the game. It doesn't always work this way, when action on big teams like the Patriots can be 75/25 the casinos get very nervous. They can have games/days that are losers, and they have to go into their own pocket to cover these wagers.
For horse racing/parimutel betting, they don't really care about the odds or who wins because they get their cut of the wager up front, and the payouts are out of the pool of money bet by players ONLY, the casino/track never has to go into their own pocket to cover these types of wagers.
