I don't think offices make great datacenters without a lot of investment in HVAC at least, beyond the handful of racks they might already have in a few rooms. Plus most businesses don't own their offices, they lease them from a property owner that would probably prefer to keep it as office space should the business fail.
So why don't they just not renew the lease on 50-80% of that expensive office space? It's not like a majority of tech businesses own the buildings their office is in. Or if they do, rent the excess out to others. (Sure, right now if everyone does either, the office-space rental market will pretty much crash... But a lot more tech corps would profit from that than would suffer from not getting good profits from renting out their property.)
