Why can’t flammable liquids and gas cylinders be included in a move?

So here's something I ran into last week while helping my aunt get ready for her big move out of state. We were packing up the garage and found a half-full canister of propane, plus some leftover paint thinner she kept for cleaning. She told me to just box them up and throw them in with the other stuff the movers would take. I had to tell her that I was pretty sure that stuff wasn’t allowed. But I honestly couldn’t explain why exactly. I mean, I know it sounds dangerous, but if the containers are closed and nothing’s leaking, what’s the issue? Anyone else come across this? Just seems like something most people wouldn’t think twice about.