Because there's a market that will pay. I was in Wal mart the other day, and they had a decent looking helmet, with a clip on rear light that was kinda cool, for $20.
Honestly, just as safe, and all you need.
We pay out the nose for very marginal weight, cooling, and styling advantages.
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You could fall off a cliff and die.
You could get lost and die.
You could hit a tree and die.
OR YOU COULD STAY HOME AND FALL OFF THE COUCH AND DIE.