How are cooperatives not incentivized to "let basically anyone join"?

The more drivers a co-op has means shorter wait times and higher geographic coverage for the passengers. If passengers have to wait too long, they will lose faith in the reliability of the co-op and be forced to use other forms of transit.

The more passengers a co-op has means drivers have to idle less or shorter (unpaid) distances to travel to pickup a passenger.

It is always better for the co-op to have more active drivers and more active passengers. The growth comes in the form of "let basically anyone join".