To your point however - I have seen other examples of caves where they did exactly that - put up a gate and locked it. I can't remember if there is actually a gate before this sign too - I just know people have died in there. So hard pass from me.
It's more like those signs in the edge of the Australian outback telling you there's no gas for 600km or the signs on that crazy ass island in The Great lakes with the 300 ft tall sand face.
Lots of people are capable and go there and enjoy themselves. The warning signs probably save at least a few people some serious trouble