They're well-funded for a tech startup, not Donald Trump levels of funding! Prime SF commercial real-estate is going for $500-$1000/sq ft these days, so buying, say, a 5-story, 50,000-sq-ft office building will run you $25-50 million. Tech VCs don't want that level of investment tied up in commercial real-estate; if they wanted to do that, they'd form a real-estate investment fund instead.