If you had an option to do so from inside the Gmail client, encryption would become much more mainstream. Techdirt [1] and ArsTechnica [2] had some good articles about it, although I disagree with TechDirt that they should offer "key management" for users. That would defeat the purpose - unless it's guaranteed to only be done by the browser, locally, and they wouldn't have access to that, and it could be easily verified that they're not lying. I think they sort of do this already for the master-password in Chrome.
I don't think these encryption options would hurt their ad-revenue much, and besides - I don't think Google, Microsoft or any other company should scour through my private messages to make their ads more relevant. I don't care how "anonymous" or secure they make it. It's okay if it's public data - but private data? No. Definitely not.
[1] - http://www.techdirt.com/articles/20121218/16095921431/why-go...
[2] - http://arstechnica.com/tech-policy/2012/12/op-ed-a-plea-to-g...
Encryption in the browser would mean no targeted ads, less profit for shareholders, problems with the government.
Again IMHO the reality of today is governments and big business are controlled by a very few people, working against the interests of the masses - i.e. a huge conspiracy.
edit: Don't they all follow what they say in their TOS? Are the TOS the same? (I don't know, maybe they are)