Because RB needs a driver that drives a similar style to Max. The points difference between Max and Checo is too much and that's primarily because the car just doesn't fit his driving style. It hasn't mattered this or last year because RB has been so dominant but look at 2021. The reason RB lost the constructors was because Perez couldn't consistently compete against Bottas. Bottas isn't an exceptional talent and the RB car was only marginally worse than the Mercedes, but Bottas was clearly able to get more out of the Mercedes than Perez could from the RB.

If next year Mercedes would have a car capable of going against the RB then Mercedes would win the constructor over RB because Perez is the weak link. It would be foolish of RB to not look for a replacement. I'm not saying Riccardo would be a good enough replacement, but based on his driving style he would definitely be more suited for the car than Perez.