i can completely understand why people dont bother to wear helmets, you're skiing, throwing down risky stuff in a park full of metal and ice, its dangerous, with or without a helmet you have to accept the risk. you can wear a back protector, pads, wrist guards, etc.. to try to make it "safer" but at some point if you cant accept that what you are doing is dangerous, dont do it.
theres also a certain level of freedom and trust in yourself, its an awesome feeling that you get when you just say "fuck it" to whatever risks there are and just continue on not acknowledging any of them. making the environment safer ruins a lot of the rush and feeling you get, i know i wouldnt have any interest in skiing if there werent a good amount of risk associated with it.
its like when you go backpacking for a couple days in the mountains by yourself without a pack full of emergency equipment, gps, a planned out route that you've given someone. it defeats a lot of the purpose, i like the feeling of complete 100% freedom and reliance on myself, that feeling and self trust/responsibility 99% of people completely lack today, i personally love the feeling, and its the only reason im even into it. something about acknowledging that life is full of risks and going out and seeking/conquering them and conquering your own fears is extremely rewarding.
that all being said, i wear a helmet when i ski, when i have one on i dont even realize that im wearing it, the thought that im a little bit safer doesnt really enter my mind, you can still fuck yourself over and/or die with a helmet. so i figure why not? but when i forget my helmet i still go out and ski just as hard.