Why don't the Olympic Games mixed doubles and singles use the same people?

Avoid association. Because of the rules of the game, the athletes of the association avoid it, so the Olympic Games mixed doubles and singles don't need the same group of people. There has always been a system of avoiding the same association in the ranking of the Olympic Games, that is, two players from the same association will be automatically assigned to the upper and lower half and can only meet in the final.