I think there may be a legal issue here. Cosmetics aside, thinner will mean less impact attenuation, and bring it below the minimum certification standards. As you say, it may not matter to you, but can to the seller of such an item. There's a concept in law of an implied warranty of fitness for purpose. Ie. if you sell it as a knife it has to cut (not very well).
Likewise, if you sell something as a bicycle ******, or that may be thought to be one, based on looks, packaging and presentation, then there's an implication that it would offer protection. If that protection is below the generally accepted standards, you'd be vulnerable to a lawsuit based on your having falsely represented the product (even if you made no representations).
Just about the only way one could market such a device is by clearly stating that it's hat, to be worn for fashion. Since few people have interest in hats that look like bike ******* no one makes or sells them.
You might find something thinner that was made for another sport, but IME these tend not to be cool, since their users aren't hammering pedals in the mid-day heat.
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