First: It's a power issue. Employment is at will so ceos feel they have more power (or at least leverage) and can say they're firing ypu for a reason. Leases have the power with the landlords so ceos feel they need to be more circumpect.
Second: it's a positioning issue. Ceos are usually thinking about their next gig. Layoffs because we overhired: agressive, good ceoing. Too much real estate: bad ceoing, doesn't look great on the next job search.