Tagging is far more difficult than it seems on first glance. There have been endless discussions, often pretty intense ones, about tagging on the whole SE network. Deciding which tags are useful often requires a deep understanding of the site and experience with the subject. And even then we often create and use tags that are later found to be useless or even harmful.
I have more than 10k reputation on three SE sites, and I'm a moderator on two. I still would not want to create a new tag on a site I don't know very well even if I could. The chance that I create a bad tag is pretty high because I don't yet understand the tagging principles the sites uses, and I don't know all the old discussions about tagging on that site.
As a specific example take tagging on Arqade (Gaming), there have been numerous heated discussions about tagging there, and the tagging principles there are not really obvious to a new user.
It's very easy to create bad tags, so a certain barrier to creating them is necessary. I even think we need better ways to review and sort out bad tags before they proliferate.