Storage is a little different since there are some things you can reveal about files while keeping them private (filetype, size, last modified time) which are relevant to providers. Email is much more difficult, with the exception of the sender and the time there is very little information that can be provided from an encrypted email. That makes it harder to provide a feature-heavy client. You of course have the option for using something more light weight; Gmail does not have that option, it would be a completely different product and business model.

Using web of trust signed keys is all well and good for techies. How would Google possibly set that up for average folks in a way that they themselves could not circumvent? I certainly could not see my parents working with key pairs unless the vast majority of the work was done automatically.