Your average successful streamer is playing Gwent for 4 - 6 hours a day on stream, and working on content or cross promotions for another 2 - 3 hours. Still, that amount of time they will easily play 30 games, which is enough to get you to 40 rounds minimum if you just win 33% of your games.
Plus if they were not console players, they were receiving random drops from watching other twitch streams, etc, in addition to whatever else they may receive from CDPR (if anything aside from the cardback.) Still, that is a ton of time to have accumulating the drops.