If Jupiter is a gas-giant then why don't its features change?

we see that its features didn't change largely over many years

Jupiter is huge. It is 11 earths across, and 1300 times our volume. The clouds/bands we can see are vastly larger than Earth's entire ecosystem and that means they have a lot more inertia.

Also consider our observation timespan. We have been watching Jupiter* for under 2 centuries, watching it in detail for under 60, and watching it in the above picture's detail for under 30. Given the scale of Jupiter's weather this is a very short timespan and we should not expect to see major upheavals within one human lifespan. Picture a nice summer day in California, from an insect's perspective - it won't change much in 72 hours, and that's a long time for many bugs.

• Galileo saw a spot of light. Cassini (the person, not the probe) didn't see all that much detail.