> It would probably pay for itself in reduced fuel/rolling resistance when going fast in a straight line on the highway.
Just having your tires inflated right at the start of your trip does that. TPMS systems will already tell you what your tire pressure is, and with so many cars ditching the spare and going with an inflator+tire goop kit the car already has an inflator you can use. As mentioned, the vast majority of cars aren't leaving the pavement, so as long as they keep their tires inflated properly its not an issue.
That little compressor inside the wheel is then adding unsprung weight, which is bad. You normally want to try and reduce that as much as possible for your suspension's sake. You'd then also need more complicated wheel assemblies. That area of the car also takes a lot of abuse with all kinds of nasty stuff kicked up in there, not exactly the place where you would want to keep an air compressor. You're also then putting this minature air compressor in the space that takes the most physical abuse in the car. You also then need a way more complicated hub design with gearing and clutches inside to properly drive the compressor when needed. This would be a way worse style of doing it compared to existing designs which just run air hoses to each hub with a central air compressor tucked away someplace safe and protected.
If I had a choice between a car that just had an inflator kit in the trunk versus one with four mini air compressors and tiny transmissions and clutches in the wheel hubs along with complicated non-normal wheels and was overall more expensive, I know exactly which car I'd take every time.