The Tsiolkovsky rocket equation. (https://en.wikipedia.org/wiki/Tsiolkovsky_rocket_equation)

https://what-if.xkcd.com/7/

  But this is just a minimum. In practice, everything depends 
  on our means of transportation. If we’re using rockets, 
  it’s going to take a lot more. This is because of a 
  fundamental problem with rockets: they have to lift their 
  own fuel.

  If we want to launch a 65-kilogram spaceship, we need to 
  burn around 90 kilograms of fuel. (Gasoline has an energy 
  per pound comparable to that of rocket fuel, so we’ll 
  stick with that example). We load that fuel on board—and 
  now our spaceship weighs 155 kilograms. A 155-kilogram 
  spaceship requires 215 kilograms of fuel, so we load 
  another 125 kilograms on board ...

  Fortunately, we’re saved from an infinite loop—where we 
  add 1.3 kilograms for every 1 kilogram we add—by the fact 
  that we don’t have to carry that fuel all the way up. We 
  burn it as we go, so we get lighter and lighter, which 
  means we need less and less fuel. But we do have to lift 
  the fuel partway.

A rocket is essentially a series of very large tanks stacked on top of each other. The economics work out in such a way that you make the tanks as lightweight as possible, and then pressurize the combustion chamber with turbopumps.

The pressures are very high (you are pumping the fuel into the combustion chamber of the rocket engine). A tank that could be pressurised that highly would be uneconomically heavy. Edit: this says it better http://www.scienceforums.net/topic/73571-rocket-engine-with-...

I'm guessing the fuel will be pressurized, and a pump is necessary on top of that to get as much thrust as you can; it's probably cheaper on a weight basis to make a good pump versus a very thick fuel container at some point.