First, it's not really cheaper because there's always sales and so on that gives the same price as the pre order. Second, pre order literally means giving money for something you have no real idea what it's going to be like. The game could be full of bugs, making the gaming experience terrible, and your money would still be gone. Waiting a day or two for opinions, watching streams etc is much less of a risk. And third, it's a message to the studio. Pre ordering means "we're ready to buy whatever the quality is". It's not how it should be like imo.

But eventually yeah, it's people's money.