The operating word is “seem,” but that’s only because you’re not seeing most games, which are small and don’t have much visibility. Indie developers are doing more merch these days, but a lot of small and medium-sized developers still aren’t doing anything except maybe a print-your-own t-shirt on some service. Although a friend of mine is part of a small team working on a game that’s sold a million copies (plus a couple million expansions) and they don’t even have t-shirts. It’s not big enough for spin-offs, but it’s also more than successful enough that they don’t need the marketing, and they don’t want to waste the money spending time on other merch.
Again, it’s about scale of audience. Most games don’t have it. Candy Crush has hundreds of millions of players.
