These big companies do not manage one single house (office). They manage a real estate portfolio. Most of big companies have a mix of rented and acquired office space. Ultimately, owning real estate or not is an investment decision.
Some big companies, like HP recently, went from owning most of its real estate to sell large chunk of it ... Reason is to get some cash flow and to rationalise some expenses. But not because someone else could manage better their real estate.
