Fair enough, but the (probably valid) assumption is that most developers already have a github account (much like most people have an email account) -- and you can then make all your changes without waiting for approval. True, your pull-request can still be refused, but even so you still have the product of your work sitting in your repo, and you can share it with others.

I'm not really a great fan of github, but it's hard to deny that it is one easy way to facilitate and mediate contributions.

Also, the permission request to edit the wiki is a one time request. Once you have permission, you are free to edit the project page or any other page on the community wiki.