I have a username

It might be that you’re on a smaller/newer instance, so your instance doesn’t know about many of the communities you’re not subscribed to.

You don’t have to — you can click “create post” at the top of the screen (if you’re using the web client at least) and select the community you want to post in. I agree that it’s pretty unintuitive that the “create a post” button only shows up under a community after you subscribe to that community though.