Italics change the semantic meaning of text too, but they're still rich text, not plaintext. Unicode has been generally taken the attitude that Unicode is for plaintext, not rich text, even if it's stretched the meaning of that a bit. (There's also the time they goofed up and added interlinear annotation, which is pretty clearly not plaintext, but they've since discouraged the use of that and I don't think they want to make any more mistakes like that.)
Sure, I could have chosen a more watertight example to demonstrate the point. (And I think that "although" is an important "although" -- those characters are not meant to be used for ordinary italics; if you do this, you're going to cause significant problems for anyone trying to process your text, such as by, say, searching it.) It doesn't have, I don't know, characters that are blue-colored bold italic underlined superscripts, you know? Whatever exceptions or seeming-exceptions Unicode may have made, they're not looking to make more of them.
