The answer is we don't. Unlike the other three gas giants in the Solar System we can't see Jupiter's pole clearly. So Jupiter is unique to our solar system, and we don't know much about the visual details of exoplanets.
The thing is, there's a lot of gas, which like any other matter, has mass. So, even though a hydrogen atom is the lightest element possible, and we think of hydrogen gas a being light, that's just relative to other things. At a human scale, it isn't possible to have that much of the stuff around. But you can always go and collect a lot of it; in fact if you get ten to the power twenty seven kilograms of the stuff, and put it in one place, gravity will shape it into a sphere that looks very like Jupiter. Or, get 1000 times as much as that, and watch as gravity squeezes it so much it begins to fuse into helium at the centre - you'll have a star the size of our Sun...!
Jupiter is almost all hydrogen. However, the bulk of that hydrogen is actually liquid. Its core has compressed the hydrogen so much that it's ionized and is essentially a metal.
