It’s true the government can do things very very well. However, it needs proper finding to do so and Republicans most of all are unwilling to spend the cash to allow the government to many effective hires at the scale necessary for something like national email.
It would also likely be a surveillance mess given 3 letter agency interests in communications. It would take a pretty significant shift in national security strategy (and possibly a time machine to elect Al Gore and/or stop 9-11) to get us out of that mess.