I think what triggered the problem with lootboxes is that the monetary value of the items is immediately obvious and you can trade those items without any friction over the internet at any moment. For many games an item is just equivalent to some amount of money. It's not a good, it's casino tokens that you can exchange for real money at the counter. Selling cards, especially before the internet, was way more complicated and most 12yo at the time probably wouldn't bother unless they were extremely lucky and managed to acquire a very expensive card "charlie and the chocolate factory" style. Cards have pretty limited liquidity, unlike digital goods that can be traded quickly and safely across continents in an instant.
I don't play a ton of games featuring random item drops but if I look at my Steam inventory for instance I can get the immediate value at which any item is trading (including loot boxes and their keys, but also skins, cards, emoticons, profile backgrounds...). Valve itself plays into this by tracking the price and making it easily available to anybody. You can directly, from Steam, trade these items for money (well, money locked into your Steam account, but money nonetheless).
On the other hand when I was a teen and played Magic: The Gattering most of the time I had no idea of the value of an individual card and checking it would have involved buying specialized magazines (the internet was only in its infancy at the time). We traded cards only for other cards and based mostly on our own subjective assessment of their relative values and how they fit into our decks (or even if the drawing looked cool), not based on some global card exchange with real-time trading.
On top of this there are a huge number of 3rd party "casino" sites for these in-game items who add another layer of gambling on top of the lootbox concept. Those websites don't hesitate to pay Twitch streamers to "play" on their site, sometimes even tweaking the odds to make them win more than average to make it even more enticing for the (often very young) watchers. It's scummy as hell and while Valve & friend don't directly partake in this they very clearly enable this behavior with their policies (and they APIs).
I could see the point of somebody arguing that MtG and Pokemon are gambling but these video game economies are on a whole other level.