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.