Frequently is a bit of an hyperbole, a lot of my games come from g2a and I never had one be revoked. Most of the time it's just people buying games when they are on discount and reselling after the discount has expired.
Bethesda games are a good example of that. You can pay 80$ for Skyrim VR or get it on g2a for 20$. This is just because the game is very frequently on sale for about that amount.
However, all of the sites listed at isthereanydeal.com I have tried are not that. It's not clear what the business model is, but they're absolutely depending on steam to deliver and maintain the content, and it's not clear that steam gets any money out of the transaction.
These keys are often stolen or pirated.
The Steam subreddit has a page dedicated to it https://www.reddit.com/r/Steam/wiki/dangersofkeyresellers
The keys from these sites are frequently revoked after they're discovered stolen.