Well, you have to consider all elements of performance, not just tire width. Even if the wheelset was optimized for aero with 18mm rubber, any gains there could—and certainly would— be overwhelmed by losses in rolling resistance, rider fatigue, bump compliance and grip.

If the riding surface were perfectly smooth (like an indoor velodrome) then those other elements don’t factor in as much, but on real roads, and at real tire pressures, it’s well established that wider tires greatly reduce rolling resistance by shortening the contact patch. Even on a smooth track, depending on rider weight, there are physical limitations to how high the tire pressures can go to get the smallest, fastest, contact patch, so 19mm and 23mm rule there.