It’s a matter of statistics. We know that it can cause a collapse, but depending on the cave it’s not often. We don’t want people to get hurt, so we’re going to warn people against generating heat like that. But with a ton of caves, it would have happened irregularly enough for people to be mostly fine.
Not an expert, but I do have some insights on this. The reason that spelunking doesn’t usually allow fires, even at the entrance, is because the smoke can damage the cave environment. Smoke, especially from things manufactured in a different environment, can contain a lot of wild stuff that interferes wjth an ecosystem. Caves, especially ones big enough for tours, have incredibly unique ecosystems that are ripe for study by biologists.
Additionally, smoke contains unique particles that settle on surfaces. We can study the materials deep in some caves like Mammoth Cave and find evidence of smoke and ash. This is strong evidence for archeologists that ancient people were able to travel deep into this cave.
tl;dr: smoke damages ecosystems and makes archeology difficult. Don’t smoke in a cave.
(would write more, but using phone keyboard)
That’s pretty wicked!