Who has time to go through 7 million accounts and determine which files are important. And they have to do this before dropbox discover the hack and force reset all accounts.
Sure there are people doesn't have 1GB of data on their account (but they have at least some data). Then there are heavy users like my company, who has 100GB+ data, 200k+ files on the account, so it balances out.
Won't they have some script or some system to go through it?
If not then, i don't think they can even download and go through 7 million accounts...
Then it's just fruitless to hack...
EDIT: I do agree that someone probably could use some kind of scrypt to go through all the files to find anything that might be of use (like wallet.dat files)