It follows the laws of thermodynamics, conservation of mass and energy. You can't just separate 2H from O without significant energy expenditure. You see, gaseous hydrogen requires about 50% more energy to create than it releases as an end product. Furthermore, turning hydrogen into a liquid (it's still about 1/3 less energy dense as a liquid then gasoline, oddly, the gasoline hydrocarbon actually has more hydrogen in it per volume then liquid hydrogen) is a process that requires another 50% of the total energy of the hydrogen created. So the creation of liquid hydrogen is currently 100% inefficient at best, and I've read that the theoretical limit of liquid hydrogen (the limit that will be the same in 500 or 1000 years) is about 60% inefficient.
I also add that the liquid state of a substance is just about as dense as it can get, it becomes exponentially more difficult to make a liquid (or solid) become more dense. For example, the perfectly spherical hollow "lens" of high explosives that surrounds a implosion method nuclear bomb can make plutonium or uranium up to 8 times more dense (it has to be insanely perfect and efficient) for a fraction of a second, then, naturally, it explodes. I If I remember correctly, only Iron can be made into an ultra dense state and actually retain its ultra density, I think all other materials return to their normal density almost immediately, but I could be wrong about this property of iron.