Cost and type of capital. Office buildings are funded with mountains of cheap debt. Pinterest is funded mostly with equity. Maybe debt later when the growth i

Cost and type of capital. Office buildings are funded with mountains of cheap debt. Pinterest is funded mostly with equity. Maybe debt later when the growth is more stable but mostly equity. Financially, equity is jet fuel. It's expensive and you use it to build startups, movies, and other intangible-heavy things. Real estate debt is more like gasoline or diesel. There's a lot more of it than jet fuel, it's cheaper, and lower-quality/energy. If you try to run a gasoline engine (office space) on jet fuel, you'd likely destroy it and would definitely waste money putting something of too high a "grade" into the engine. Likewise, you can't run a jet engine (startup) on diesel. It won't run. Getting back to the question at hand, it's more efficient to run the offices on cheap real estate funding, and have the Pinterest operating company lease the asset, than have it own the office directly. Sometimes companies will buy offices themselves (like Apple) but it's something investors put up with, not encourage.