Why is Composing Message Format only Rich Text

As per this article : If your email contains a winmail.dat attachment - Apple Support

You can use a third-party utility to open the attachment.

Messages sent from older versions of Microsoft email apps, like Outlook or Microsoft Exchange, might include an attachment with winmail.dat in the name. The file doesn't appear in the sending app, but it does appear in Mail. In other email apps, it might appear as a MIME section named application/ms-tnef. This happens when the sending app uses Transport Neutral Encapsulation Format (TNEF).

The winmail.dat file might not contain any useful information or it might contain a document or other file.

To open a winmail.dat file on your iPhone, iPad, iPod touch, or Mac, follow these steps:

1. Open the message in Mail.
2. Touch and hold the winmail.dat attachment. Or on a Mac, choose File > Save Attachments to save the winmail.dat file.
3. Open the file with a third-party winmail.dat viewer. You can find these types of viewers on the App Store.

Rich text of course supports bold, italic and color... not really clear what you are saying here.

I don't use Outlook so I can't answer the second part, except that if you receive a winmail.dat file that is determined by the sender, not the receiver.

I don't quite understand the first part of your post. But for those winmail.dat files are various (free) apps available, e.g. this one: TNEF's Enough on the Mac App Store

the choice is Plain Text, or Rich Text.

Mail is not a textprocessing app.

You can have as much choice as you want with a good textprocessor, then save that text and mail it as an attachment.

Lex