why do people wear helmets when they snowboard?

I have hit my head a few times without a helmet on. Once I hit some ice on a double black diamond in Vail and knocked myself unconscious. The piste was empty, as it was early season not much snow and a difficult piste. I eventually bought a helmet and wear it, especially when it is icy. Sometimes on soft snow and easy routes I do not bother, but off piste and when rocks and ice are around I do.

Also I have banged my helmet on ski lifts before, do not forget that much damage can be done by those machines.

You have obviously never ridden on man made groomed snow.

Let me guess, you wouldn't wear a helmet on a dirt bike in sand because it is soft?

Snow is less soft than you would think most of the time. When you are moving fast, you fall hard and can easily get a concussion, even if all that your head hits is powdery snow. In addition, most snowboarding happens on packed snow, not fresh powder. Sometimes the snow starts to melt and re-freezes, forming a layer of ice. It is often not very soft at all.

But beyond the snow itself, there are other hazards for a snowboarder's head. You might run into a tree or a pole if something goes wrong. If you're moving fast enough, that can kill you. Although it's rare, most of the people who die while skiing and snowboarding die because they hit a tree. More commonly, it is easy for a beginner to fall and hit themself in the head with their own snowboard. If you fall forward and your legs swing up, it just happens. Having a helmet there helps a lot.

Helmets are designed as a second skull to help protect the brain. These are no guarantee against fracture of the skull or a concussion BUT they will lessen the amount of damage. Riding without one is a risk that should be avoided.

Helmets are worn to protect from impacts from 1) Hard snow and ice 2) trees, chair lift poles, and rocks 3) and especially from impacts with other skiers / snowboarders.

Helmets for snow sports gained widespread attention and increased acceptance when actress Natasha Richardson fell on the snow during a beginner ski lesson and suffered a fatal epidural hematoma

hitting trees has killed many a snowboarder

They may be compulsory in competition.

Snow isn't soft. Ski and snowboard runs rarely have powdery conditions; usually it's been machine groomed or is packed down. If it gets icy, then it's like concrete. Plus, if they're going at any kind of speed they'll slide into something (tree, gate, another skier/sledder).

It's what smart people do because they haven't incurred a head injury that could affect their ability to logically and rationally think.

Snow isn't real soft when you hit it head first at 30 mph or more.