While I agree with you for the most part, according to this[1] Musk does seem to have a mansion. It says the value is only $17 million though, so pretty far from Gates[2].

[1]: http://www.bloomberg.com/news/videos/b/6e27fcba-309d-494e-b8...

[2]: https://en.wikipedia.org/wiki/Bill_Gates'_house