Because a hackers time is valuable, and google has solved my email requirements. I dont want to set up and maintain a email server. It can be annoying as hell I've done it before. It's 'fiddly'. So many config files(If you set it up properly so it uses encryption) and a lot of email services will block you if your not on a whitelist or sometimes blacklist you for silly reasons. I used to run my businesses email on ec2, you will be going through blacklists and whitelists for a long long time.