[rags2riches]

If one leaves it up to the law, it's easy. If not, it's basically an expression of personal weights assigned to a whole lot of fuzzy things.

Can you draw a clear line between when you wear a helmet and when you don't? Is it perfectly obvious or more up to habit? Would it be influenced by a change in laws? Or if there were helmets available marketed for the specific activity?

Is it obvious where riding a bike fits in between riding a car a car, climbing a tree, playing soccer with the kids, having a shower, walking up a staircase, cooking in your kitchen, walking near a pool, hiking in the hills... Your head can be hurt in so many ways. Wearing a helmet might save you. So why don't you do it more often?

[ghaff]

At least among certain demographics in the US, a very clear orthodoxy has emerged between uses where you must wear helmets vs. where helmet use would seem pretty silly for the most part. There's some logic in the categorization but almost certainly not the clear line that some people would like to think exists.

[shreyansj]

That sounds like a strawman argument. If you don't wear a helment while doing activity A, why wear it while doing activity B.

To answer your question, perhaps because the common factor in all of these activities is that I am moving relatively slowly and my surroundings are basically stationary. These factors are void if I am riding a bike (or a motorcycle), where I might be moving at a few mph and there could be vehicles around me that might be moving even faster.