I'd imagine its because its one thing to make a basic rigid shell (able to hold its own weight without collapsing) and another thing to make a giant pressure vessel able to withstand immense pressures. Imagine a storage tank made out of glued together popsicle sticks with a plastic bag around it versus a CO2 cylinder. One is going to weigh quite a bit more than the other, all because one is trying to fight some massive pressure differentials while the other can accept near ambient pressures on both sides.