Flying regularly into SFO, the shuttles make very little sense. It's far cheaper to take BART and then walk. It's not much more expensive to take Uber or Lyft. Both are far faster by the time you've driven half way around the city. And the vans are so ancient that I very much doubt that they'd pass even rudimentary safety checks. I can understand why they might be used for far flung places, but then the economics for the operators don't work.
I suspect that the real reason for their continued existence is the commissions to the hotel concierges - most hotels really push the shuttle services.