Because that wouldn't be compatible with existing IPv4 deployments and would cause a reliability havoc when a node not configured to deal with it mangled packets transparently somewhere in between your source and destination end-points.
I don't get the resistance against IPv6. It works. It's a fresh take. Yes it requires some new stuff to be deployed and configured here and there, maybe even requires you to learn something new, but if you thought extending IPv4 would have been any other way you are deluding yourself.
If you're going to do a significant change to something as big as the internet (and introducing a new address scheme is that, no matter how you implement it), you might as well step back and think it all through instead of applying yet another hack.
So tell me. Why are you opposed to IPv6? Why do you want to hang on to this old IPv4-thing which is already at the bursting point, at the edge of what it can take?