Polls are crap
It’s a little bit notable how they change over time, but mostly it is just a bunch of poor methodology noise
But, I do feel it’s relevant to point out that even in polling land Biden has been gaining for the last few weeks and it’s notable which media outlets are pretending it is the opposite of that (like severely the opposite) and to wonder why that is
They don’t though, they only care that the money bet equals the money paid out minus the house cut (juice). When the line changes, it’s not because the casinos believe one side has a better chance. It’s because the money being bet on one side is uneven. Unlike their customers, casinos don’t like gambling their money.
?
That’s not how it works
You’re betting with the casino here, not directly with other users. When you bet with the casino about who will win the election, the casino’s “house cut” is baked into the non reciprocal nature of the odds on the two sides (plus whatever fees). If they have the odds wrong enough, then their expected value can go negative. As you said, they don’t gamble, so they do their damnedest to make sure that doesn’t happen.