In "The UNIX Programming Environment" from 1984, mail is described like this:
"Sending mail to someone is straightforward. Suppose it is to go to the person with the login name nico.
$ mail nico
Now type in the text of the letter
on as many lines as you like...
After the last line of the letter
type a control-d.
ctl-d
$"

Writing an email and sending it are pretty different functions, it's doesn't looks like that because the sending part is just pressing a bottom and all the protocols and sever communication is done in the background hide from the user.

It's like asking the postal service to also write and package your mail and then open and read it for the recipient.

I personally like that philosophy and try to use it in my job, with functions that only do one job and programs that only do one job and then just chain them and make calls from each other. My users doesn't know it's like that and they doesn't notice any difference, but it's help me debugging or implementing new functionalities.