Which makes sense, but reading emails to provide smart features is not an automatic loss of privacy.
Google runs the SmartBot2000 software for you. If you use FOSS but run it on AWS EC2, you still need to trust the software, manage the upgrades (or trust the auto upgrade feature, trust the maintainers), and trust AWS for not fucking with your VM.
We have a few physical servers. And probably always will. We run our own email, but also use gmail too. Because sometimes our email breaks, sometimes [understandably fewer times] gmail breaks (or is not available, such as in China). And we will probably always run our email, but it's a lot of work, and it's not for everybody. And even though postfix, dovecot and K9/thunderbird/roundcube are all fine, they are not as smart as gmail. (but usually snappier)
You need decades of experience to set up a secure email host. GMail does it for people in exchange for showing them ads. (uBlock FTW, BTW)
NSA/Prism is a valid reason to use your own mailserver, but then again, keeping up with TLS/OpenSSL issues is a valid reason to use a non-self-hosted solution (as they do it for you, though gmail accepts unencrypted SMTP :/).
And of course, at the end of the day, there's ProtonMail!