Well, you can now. Here’s how you do it:

+ text +
(remove spaces)
text

Underlining is not yet visible in the preview box, but becomes visible once you post the answer. I’ve decided to enable underlining because I felt that it was something basic.

Also, use underlining sparingly. In most cases, italics and boldface look nicer than underlining. Answers with too much formatting can be considered low-quality.

My malevolent side wanted to enable highlighting, but that would have been downright evil.