OP's probably referring to induced demand [0], or something similar to Braess's paradox [1].

But I see your point though. If people are incentivized to share a car then that might reduce the load.

That said, my suspicion is, once you take the driver out of the equation, you can do so much more than just ferrying people. E.g courier/shipping services, or mobile food stalls (food truck that sells food while moving on the highway, once engineers figure out how to safely and autonomously speed match two cars), etc. The induced demand may be greater than the savings from ride-sharing.

[0] https://en.wikipedia.org/wiki/Induced_demand [1] https://en.wikipedia.org/wiki/Braess%27s_paradox