I can see three positions aside from yours. You are making the assumption that people know what they are getting and actually want to keep the game after they receive it, and they are acting as beta testers. Actual pre-release players may get a chance to influence the potential changes to the game, so there is also that.
You will also have the camp of people that want to be the first to play a game, just for the sake of it, or in the case of perpetual gameplay, an actual advantage in getting in early. This is basic FOMO.
From the developer/consumer side of things, pre-orders and sales lets the developer pre-allocate money for game patches and improvements. If they get no pre-orders, they may consider it a lost cause and put minimal money into the product post launch. In that case you could say that pre-orders may come from people who want to see the game succeed.
On the other hand, I myself prefer waiting for some reviews. A lot of game launches have been all hype lately.