Not everybody, no. It's like... if you have Water Tribe heritage, there's a chance you could be born with the ability to waterbend, but it's... well, I wouldn't say a small segment of the population, but it's definitely smaller than the amount of people who can't bend. The other nations are like that, too. If your family is from the Fire Nation, then you might be able to firebend, and if your family's from the Earth Kingdom, if you were a bender, you'd be able to earthbend.