Because you - like me - will probably be happy to be bought out for sums of money over a certain number. (let's say, 1 billion). And unless you have a fairly hardcore contract with me, I'm not willing to bet your service won't be wound up by the acquirer.
So my pictures are on my hard drive and an external drive.
I have a half-written blog post from a few months ago (proof: http://note.io/1lde2HC ) that tries to address this. My idea was that every SAAS startup should start with an exit plan for its customers that would be legally viable in the case of acquisition, being shut down, or any other scenario. Basically a set of software tools and policies that are clear from the get-go, so you aren't waiting to see what they'd do in that case.
We're working on some pretty major releases here so I haven't gotten a chance to finish the post (and publish a plan ourselves) but it's something I think about a lot.
It's a SAAS problem beyond the photo space.