John D. Rockefeller started the University of Chicago. Much of his time had to be spent convincing his trustees that they had to spend time soliciting funds from other donors than him. They tended to get slothful and spendthrift otherwise.

Teaching isn't presently scalable. You can look to the failures of our public school system for evidence. One might be tempted to look to other countries for systems that work better, but there has to be a political path forward for implementing those systems here.

One good teacher can only teach so many. That good teacher may not be good at teaching teachers. Teaching is not warfare, where good soldiers can teach other people to teach soldiers.

Teachers that are good at teaching teachers can only teach so many teachers. Any attempt to scale a working method will eventually run out of talent. And if you don't stop when you run out of talent, then the quality of your teaching will inevitably go down. Universities can only get so big.

Billionaires who want to put their wealth towards education are pretty limited in their options.