Which were originally included in packs of cigarettes!
* There's no "free". You must spend money at the outset in order to play. This sets precedent and expectations.
* There's a physical good involved. You always get something, and you've got total freedom with how you use it. You can play it, sell it, trade it, turn it into an origami crane, use it as a proxy for a different card, etc.
* Because people can sell cards, this means you can simply buy the exact cards you want without resorting to randomized packs.
* Buying a randomized pack is a much more involved—both physically and psychologically—process compared to an IAP. At your local game shop, you have to go up to the counter, ask for one or more packs, and hand over your money/card. If you want more, you have to do it all over again. With an IAP, you just tap the "OK" button.
that's actually one reason TCG markets don't get cateforized. They don't have a "market price" for any one card. They let the fans set that. There are plenty of 3rd party market places where you can buy a specific card but you'll never find the company endorsing them. They only sell booster packs for a set value. They can give lists of what's in a booster pack and even categorize them by rarity, but they never actually give the card a price.
* Once the bottom falls out, bag-holders get burned, but the collective nightmare is over.