Penn's budget is $4.7 billion (just the university, not including the hospitals). Even with a $22 billion endowment, they can only fund a fraction of that off of investment income.

And what are you even talking about "coming back to the taxpayers"? This isn't like a sports team holding a city hostage to get a new stadium. They apply for competitive grants to do particular research projects, then they do those projects. They aren't asking for a handout, they are being paid to provide a much-needed service (health research).