Why Gerrit is unable to create branch by itself?

The accepted answer is referring to a feature that allows users to create branching using SSH, all it adds is the CreateBranchCommand. The original issue request might actually refer to what @trejder wants but the implementation is just a create branch through an SSH command.

I was under the impression that if you have the create-reference right you can push to ref/for/new-branch but I was wrong, just tested it and it doesn't work. It only allows you to create new branches but directly pushing to them.

Guess the fastest way to get it done is:

git checkout master
git push origin HEAD:new-branch
git checkout new-branch
git push origin HEAD:/refs/for/new-branch

This feature was implemented very recently and will be available in Gerrit v2.9.