Because they are not made of very combustible gases. Most of their mass is hydrogenum and helium (up to 95% of the total), mainly not gaseus at all but in liquid or metallic form (due to the high pressure). When their size reaches about 13 times that of Jupiter, the inner combustion process becomes significant (they burn deuterium). But at this point, they are already classified as Brown Dwarves (protostars).
I was thinking that since hydrogen is such a [volatile] element and Jupiter is 90% hydrogen that its like a bomb. What if a very large asteroid hit it then the explosion from that could be like a mini sun because the asteroid cut cause the core to crack? By the way this is all dreaming stuff.
Hydrogen is not so easy to take fire, if not we all would be walking bombs. Our body is mainly made of water (H2O), and oxygen is just the perfect environment for combustion. The flammable part is the deuterium, an isotope of the hydrogen, which is not so abundant in gas giants. An asteroid hitting a planet like Jupiter would not do any sensible harm: that actually happens quite often in reality. A collision with our planet, for instance, would be for a celestial body like Jupiter a major event, but nothing catastrophic. Gas giants don't have a solid core that can suffer from the impact and, being fluid masses, are highly resistant to shocks.
You sound like a scientist, is that your background? I guess its more likely for a planet to go cold that gain heat unless it were very close to a star. I wonder if there is a planet that could be a mini sun just full of gasses and heat but still having a structure to it. When you think about the sun its just amazing to think about all that constant combustion happening.
No, I just made a research on Wikipedia after reading your post because I was wondering the same thing (laugh)ha. They are cold outside but always very hot inside due to the very high pressure. There are planets that are mini suns, they are the Brown Dwarves, super big gas giants.
Ah, you 'cheated'. Seriously though its marvelous to think about how things are. I can see a spaceship in maybe 2 or 3 centuries from now probably going up to one of these gas planets and refueling from the atmosphere!