Ironically, I had Amazon flag and undo some gift card purchases (of cards, not with cards) that I made for Christmas, while myself thinking about this category of problem, about why cards are a mechanism for scams rather than specifically money laundering.

The cards were to family members that I normally send gift cards to at Christmas, and the activity was counted as "sus" even though I was asked to validate my card number and expiration date before being allowed to make the purchase.