It's because James Simons is a serious and actually intelligent person with depth of knowledge, and thus, he is one that doesn't need to opine on random things. He also makes a lot of money in a very secretive way. I'm not sure why anyone expects him to expound in detail on how Renaissance Technologies does it. Simons also spends more money than some governments on very serious and advanced scientific research. I suppose he's rather busy with all his investments and philanthropy. Musk is busy selling himself and filling his unfillable ego.

You can see in Simons' TED interview with Chris Anderson how he gets rather annoyed with Chris at times, particularly when Chris mentions Simons' "amazing mind" at around 8:30. Simons seems, to me, to be a rather pragmatic and humble person who doesn't particularly enjoy flattery. Chris even throws out a quote by Musk at the very end of the interview, fishing for some similar platitude from Simons, but Simons deftly avoids giving out such an empty quote.

https://www.youtube.com/watch?v=U5kIdtMJGc8

Most interestingly, you can tell Simons' like of science and mathematics and alludes to his interest when he mentions his investment into the origins of life towards the end.

There are two interesting talks or interviews I know of with Simons:

* Full-length Numberphile interview: https://www.youtube.com/watch?v=QNznD9hMEh0

* His lecture on his life given at MIT: https://www.youtube.com/watch?v=SVdTF4_QrTM

Simons actually attributes a lot of his success to luck.