Generally Unicode is for encoding all existing encodings/writing.
So you generally can’t add something because it would be cool or fun or useful, but only because it is currently in use and cannot be encoded by Unicode.
That's not at all the case. Unicode began as a standard for making things like string(':)') in to a single character.
Consider all of the languages it supports. Consider: ﷽ (which isn't an emoji, but the point stands) which is an entire sentence. It was already in use in certain places and unicode decided they wanted to support it, so now they do. Previously, one would have to type out the entire sentence in the original characters, but now it is a single unicode, just like u+263a () used to be alt+1 (). The emoji was already in use long before unicode existed, and in seeing it in common use, they decided to support it.