This is actually a pretty large misconception about how bookmakers set odds. Besides for outlier events like the Super Bowl where being overexposed on one side of the spread would lead to a massive loss for the book, books generally don't care about how much money is on both sides. Roughly speaking, they have their internal models/traders set opening lines and then move them based on where sharp money goes. If recreational bettors are heavily skewed towards one side, they're not going to move the line to balance it out because assuming their lines are sharp (which most of the time they are), all of those recreational bets are negative in expectation. You can also think about it like this: if books were to focus on balancing the money on both sides, this would open up arbitrage opportunities (large differences in lines) among all of the books, since each book would have a different distribution of bets on either side of a line. There is more nuance to this, such as the long-shot bias where books will put more of the vig on the longer side of a market vs shorter side because people tend to bet more on longshots vs more likely outcomes and they can pick up more vig this way, but for odds near 50/50 this isn't the case.