Plus there's people in Europe talking to each other that don't want the USA to read their emails. I mean there tons of situations wheee emails don't go through Google or Microsoft.
For the use case of you and your wife, you don't really need to self host a mail server, you just need a file server, unless you're forwarding each other emails you got externally, in which case, someone probably already read it.
My overall point is that email requires at least two parties to have access to the content, oftentimes more than two, and chances are one of those parties is not privacy conscious, which breaks any privacy you think you might have.
So if they self-hosted more they would t go through the USA.
And I bet now that 75% is decreasing rapidly.
But really email is not secure. It should be encrypted on the wire server to server but that's not guaranteed.
So just try hosting it yourself. It's not a big deal.