It's because of their Simons Foundation support, but not only because of that. I mean, I invite anyone to name another billionaire pet project of comparable quality.

Good game and a hard question, especially if you make "comparable" more explicit. I'd add "noncommercial, open-access", and "modern" in the sense that it happened under the current norms with respect to legacy and the social contract.

I like what MacKenzie Scott is doing in the space of "What it takes to actually spend down billions of dollars"

Carnegie Libraries, Nobel Prizes, Rhodes scholarships?

Clay Mathematics Institute, with its 7 Millennium problems

Mathematica?