Because valuation of a software company doesn't come from the products they offer, nor the current user base with credit cards on file. It comes from a perceived potential growth/platform/network effect so all of these companies have to pretend that they will someday be a panopticon that commands your entire life in order to keep that sweet VC money flowing in even if they just repackage s3 with a nice client.
Woah. I clicked the link and was logged in already. I'm sure I logged in on this computer (work) at some point, but I'm disappointed that session didn't expire. I can't remember the last time I logged into dropbox here, it feels like months!