The world needs a driving licence for email. It would mandatorily include use of plain-text and bottom-posting.
I used to think the same but I caved when I started using web interface email clients. Let’s face it, the world has moved on, new generations are online, and we are the wrong ones now.
However, the original email is included as a convenience in case my MUA doesn't support threaded display or it's a mailing list I joined after the original email was sent or any other reason why I might not see it. That's why there is quoting at all.
Nobody top-posts when using selective quoting because obviously it's different.