Nate Silver has a financial and career interest in betting on campaign polling. Runaway races are not profitable to bookies and casinos. Long odds in any direction present huge financial risks for the bookie. Odds and spreads are set close to the point where the bookie's profit is equal for all outcomes, so there is no systematic difference.

However it's hard to equalize bets on events that are predicted to be unlikely, with long odds. Bookies face losses if longshots come in. So promoting the idea it is a close races, where the odds may shift in small amounts — and can be easily pushed to a perceptible difference by not much effort —  are much more profitable. Signed, great granddaughter and granddaughter of reputed Chicago sports bookies.