I think the reason there's no other popular standard is you give up something. 128 bit gives a pretty low risk of collisions in almost all uses, but as you go smaller you start having to consider the specific scenario and impact, etc, which doesn't work well for a standard.
You could use another encoding (eg base64 or base85) to get it shorter, but you start sacrificing other things (case sensitivity, url-safeness) - again, not great for a standard.