Why is it that italic and bold formatting is available while underline is not?
I guess it is voluntary, I'm just wondering why that would be the case.
Underlining as a typographic means of emphasis is a relic of typewriters and handwriting. I'm not saying these two are dead (at least not both of them), but underlining is far inferior to bolding (for emphasizing and having it stand out from the rest of the text) and italicizing (for emphasizing but leaving the text's gray value intact, thus not standing out).
Using underlining for emphasis is only for when you have no other way. And we certainly do.
In addition, Juhana is right; on the web, underlining means "clickable".
You can u̲n̲d̲e̲r̲l̲i̲n̲e̲ t̲e̲x̲t̲ by using the unicode combining character
Use this bookmarklet to underline the selected text in the active input control: