How do you run an efficient server-side search without maintaining a parsed index? I have 40,000 emails in my inbox. You want this to be linearly scanned everytime?
At best, you could search headers if they aren't encrypted.
Until a viable ciphertext search scheme arrives. (there are some, but everyone I've seen has some caveat or hole)
I don't think users actually care though. Google's attempt to redefine "end-to-end encryption" to include decrypting something mid route bothered me. This... meh.