[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.