Tanzania, Aug. 26 -- The gambling industry spends enormous sums chasing novelty, yet its steadiest earner is a wheel designed in eighteenth century France. Roulette has outlived every fashion in the business, and the reason is not nostalgia. It is accounting.

Understood as a product rather than a pastime, roulette has a set of commercial properties that almost nothing else in entertainment can match, and the move online has only sharpened them.

Every bet on a roulette table faces the same built-in margin, created by the gap between the payout odds and the true number of pockets on the wheel. On a European wheel that margin is about 2.7 percent of everything staked, spin after spin, forever. Compare that with sports betting, where one un...