My pet theory is that laughter isn't "just" a play signal, but a teaching method in social animals where the young learn from adults.
Slapstick is popular for a reason. We laugh instinctively at others' accidents because being laughed at provides a negative reinforcement for the individual that is the object of ridicule.
Being laughed at is not pleasant, at all. It's not "fun" or "play". It hurts. Most people will do anything to avoid being laughed at, especially in public or social situations.
That's... the point! Being laughed at is negative reinforcement, teaching us what not to do. The reason even our friends and family will hurt us emotionally in this manner when we make a mistake is not because they're cruel or mean, but because this is the mechanism the human species uses to make sure every tribe member learns the lessons they need to learn to keep each other safe.
That's why we all think it's funny if someone slips on a banana peel. We point and laugh as the victim is crying in pain.
We don't think it's funny if a banana tree falls on someone. We don't laugh. We run over to help immediately.
In the first case the person wasn't paying attention and needed their inattention corrected. They were at fault and needed to be taught how to walk safely, like a young child.[1]
In the second case they were not at fault and laughing wouldn't help them improve. They just need help.
If we didn't find laughing at silly people pleasant, we wouldn't do it, and then... people would learn less and make more mistakes. They -- our children or cousins -- might even die or accidentally kill other tribe members killed through their ineptitude. Hunting is lethally dangerous. Mistakes must be punished. Laughing minimises mistakes in our genetic kin and tribe members on whom our own reproductive fitness depends. Hence, laughing would be highly selected for in intelligent social creates like apes and especially humans.
PS: It's a fun exercise to think of more scenarios where you would laugh at someone or similar scenarios where you wouldn't. You'll find that much of the time you would laugh, the person you're laughing at would have probably learned a lesson from that that's beneficial to you. (e.g.: would keep you safer if they made less mistakes in the workplace around dangerous tools.)
[1] Young children are especially hilarious to adults precisely because they make so many mistakes that need correcting! We laugh less at adults because there's a delicate balance between hurting people emotionally versus the expected benefit of the lesson being taught. Similarly, senior people will laugh at the mistakes of junior people, but the other way around is very rare.