Cost is absolutely an answer. If I'm not really using space and am completely indifferent to whether I have a permanent desk or not, why on earth would it be rational for the company to spend the money, time, and disruption associated with adding additional real estate?

I'm not arguing for hot-desking as a standard approach, which indeed likely negates any benefits to teams being co-located. But for people who are allocated space that they rarely use? Why not?