It is up to the vendor. Some don't credit because the items are from elsewhere and they can't. Sometimes I don't credit because I use a mish mash from various places, but I always tell people when they ask what I used. There has never been a rule in place. Perhaps most of the items you are looking at now, the additional items came from elsewhere?

Ever since that credit option was put in place I see more products without credits than those that have had them. It's more likely just the ones you are viewing now.

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I do not feel obliged to believe that the same God who has endowed us with sense, reason, and intellect has intended us to forgo their use.
-Galileo