I think the key point here is that - if a founder is raising money for a company from investors, and the founder simultaneously buys a mansion - the founder is implicitly communicating that s/he doesn't believe that investing that capital in the company is as valuable as a real-estate investment in the mansion.

Founders have a very specific job to investors: to return their money with a hefty return on investment. If the expected ROI is higher buying a mansion, then maybe the investors should invest in mansions rather than the tech start ups.