Remember, Red Bull is first and foremost a marketing company that sells stuff.

I suspect he’s there because Red Bull sells a lot of cans and merch in Latin America with him on the team, and also because Carlos Slim sponsorship is worth a lot.

I think they also know the Drivers’ Championship is more important than the Constructors’ Championship for marketing purposes.

I suspect they’ve done the math and know that he’s worth more to them on the team than someone who would be a better driver.

(Also it means they don’t have to worry about the number 2 driver challenging the number 1 driver ever. The cost is that they also can’t use him to defend against rival teams, but that hasn’t been a meaningful problem for them yet.)