Isn't this easily defeated by closing the betting before the wheel starts spinning? Is that not standard practice anyway?

Yes this would defeat it, but standard practice is that you can keep betting a couple rotations into the spin. The reason for that is that some people believe the croupier can target his shot to some extent. (I don't think it's farfetched that someone who spins a ball 8 hours a day could get above chance at targeting.)

Leaving the bets open while the ball is spinning causes a lot of players who think they can time the ball (or as another commenter said, believe the dealer might be cheating) to bet and presumably yields higher profits.

There are other countermeasures developed nowadays, such as Cammegh's RRS (Random Rotor Speed): https://www.cammegh.com/our-products/roulette-wheels/mercury... - essentially after bets are closed the wheel is able to ever-so-slightly slow down at random times, throwing off any prior calculation.