Here’s a dev explaining it: https://lemmy.ml/comment/2618947
Apparently they do chargebacks, which costs the gamedevs money.
This is something that should have been in the opening post.
It explains why using these sites actually causes harm.
Instead of getting a game at a reduced rate without harming the dev much (just losing a sale) you’re actually harming the dev.
This is something I didn’t know and now I’ll look more at discounted games on official platforms instead of these key sites.
That’s why I stopped using those sites. The only reseller I buy from now is Humble Bundle, but most things I just buy direct from the Steam Store.