Why are firebenders only in the fire nation? If there's actually more then why don't they make their own nation?

I'm not a firebender expert but I know in the Fire Nation it's like 20% can actually bend there, so I could imagine there to be a lot of other nations with firebenders in it. I just don't see why they wouldn't make their own nation like the Earth kingdom did when they found out it was like 25% on the planet could bend earth.