Unless you live out in the middle of the wilderness, you’re not likely to go far without seeing a sign telling you to do something.
Both their ubiquity and their messaging usually mean they’re not particularly noteworthy since what they’re telling us not to do is often something we weren’t too excited about trying anyway.
Obviously, the fact that they exist means someone thought it was a good idea to mess around inside a power transformer or dive in the shallow end, but that’s simply not true for most of us.
So for a sign to actually get our attention, it has to do things with a bit more flair. And if it makes us laugh, it means we’ve proved that it’s noticeable.