One idea, at the scale of large companies, a single building isn't really that big of a deal. Most medium size and even smallish-large companies have empty offices, empty floors and more. I can think of half a dozen companies I've been to in the last few years with whole floors in their buildings that sat unused. At a large scale there's no reason an entire building wouldn't sit idle. It only seems weird because this building has obviously sat this way for a long time.

Look at it this way, suppose they decided to lease the space to another tenant - say for 5 years. Market forces could change a year after the lease is signed and they'd suddenly need the building again. But now they don't have the space anymore and need to get yet another building.