No, it's an actuarial model. Priority Pass is owned by an insurance company. They know how to set the cost of membership (individual, corporate or bank sponsor) to recover their costs based on average usage projections. Individual usage isn't very important if the number of members is large.
Another example of a business model like Priority Pass' is streaming video. The key for Netflix and similar is to charge customers enough to cover their costs (mainly the amount paid to the studios for each movie/TV show played and the infrastructure to enable streaming) but not too much where it dissuades people from signing up. This is possible because nowhere near all of their customers are going to constantly be streaming stuff.
That said, I'm sure more people than expected are using PP, hence moves by various issuers to limit free guests and otherwise control usage.