Hi @erica_wagner ...
I think we all know it's not email, per say, but a messaging system within canvas that allows us to send messages to the students in our class sections, including individual messages. It's very handy, since getting each student's individual email and creating an email list for each class by hand is cumbersome and time consuming.
Not sure I'd totally agree with you that everyone knows it's not e-mail...but I wanted to make sure to clarify for anyone that might be reading this particular question and anyone that was a New Member (such as @CarlaLawrence) since she might be new to Canvas.
The question is why the within canvas messaging system (which isn't email) doesn't have the same functional text editor and ability to link to material within canvas as the other editor-based features within canvas. For instance, I need to link a specific file to one individual student. Linking it in a general announcement would create confusion, as the rest of the class does not need this link. I can't find a way to do it.
I cannot speak to the "why" question. I recall several years ago in one of the (then called) Feature Ideas that Scott Dennis from Instructure had chimed in with an explanation about why the RCE was not part of the Canvas Inbox. I don't know if his posting is still available today or not. However, I do know there is this idea in the "Ideas and Themes" area here in the Community that relates to the request for something like this:
[Inbox] Add rich-content editor to Conversations - Instructure Community (canvaslms.com)
and
Improve the inbox functionality to provide for mor... - Instructure Community (canvaslms.com)
You might want to follow the conversations in these areas...as it looks like it's been identified as a need.
Hope this will help a bit.