I believe the Reply-To name is dictated by the person's address book, not dictated by Marketo which is why you can dictate in incoming name, but not an outgoing name (only to which address it should be going).
Mmmno, it's because the Reply-To has very complex semantics for a friendly name (display name) in the SMTP specification (RFC 5322).*
That's why it's not offered in the email editor, because the simple friendly name is associated only with the From:.
* For practical purposes, there is no display name for a single Reply-To address in the way you're expecting.
On the off chance that someone would object to saying "there is no display name" I chose the more precise wording.
I think it will be very counter intuitive if name of the person changes while replying. For example - if you get a mail in your outlook and you decide to reply, then it is expected that the reply to name will be the same from whom you have received the mail, right? if the name changes then it will be kind of red flag.
Also as Sanford mention about the technical aspect that this option is not available.