How does fire create thrust in rocket?

What the rocket does is holds the propellant in a lower-pressure, easier to store form, with higher energy density. Basically you have a lot of that kinetic energy you'll need in the form of chemical energy. Then you can pipe that fuel to the combustion chamber and ignite it there. At that point all that chemical energy is released in the form of expanding gases and heat.

So, instead of having a very heavy container holding all that hot pressurized gas, you just have a smaller container holding your fuel.