I mean they kind of do. Most of the time I would hand wave away any company offering gift cards or credits, but Steam has created an economy / structure that I think warrants mentioning here.
I have sold a few items on Steam because I don't care about cosmetics in games. I'm also lazy and because of that "sat" on items for a while that appreciated. I mention this because Steam credit is very fungible: it can be easily converted.
Steam also makes it very easy to redeem credit, gift, etc.
I believe you can buy Steam cards at most places Xbox cards and similar are sold as well.
Also in the early days of Bitcoin buying and selling of digital Steam assets was one of the most popular things.
On the other hand, I'm absolutely amazed some US states hasn't yet gone after Valve for running an unlicensed casino with no age verification.
I think loot boxes as a whole need to be regulated as they are clearly gambling. I'm not a fan of regulation as a solution to most problems, but when it involves children I think it sets a good framework for safety and if someone wants to start gambling later they are free to do so.
Valve goes one step beyond loot boxes with their marketplace: you get a loot box, pay to open it (basically a slot machine spin), and you get an item in a "game of chance" -- but that item is a "thing of value" that can be sold on Valve's official, first-party marketplace.
Every definition of gambling I've seen includes some variation of "winning a thing of value from a game of chance", and while loot boxes for in-game rewards skirt that (the thing you're getting is typically described in the fine-print as having no monetary value), Valve's user-driven marketplace is setting a real-world value.
Put another way: EA FC's card packs or Genshin Impact's gacha spins are worth nothing outside of those games, but I've sold Counter-Strike skins to pay for a decent chunk of my Steam Deck.
I know that physical Steam gift cards exist but I've quite frankly never seen them anywhere. Nintendo/PlayStation/Xbox cards are pretty ubiquitous though. I recently tried getting a Steam one from a grocery store but they only had the console ones.
I've definitely seen them. A quick search shows them available at BestBuy and Walmart at least.
I'm not American so I've never stepped inside a BestBuy and Walmart. The last place I checked was a Lidl, where they only had the console ones.