Everyone knows about The Witcher and CD Projekt Red at this point. Hearthstone does just fine without being on Steam. The Elder Scrolls: Legends isn't on Steam either nor are there any plans for it to be in the future.
While what you're saying is true, you can't really compare the size of Witcher/CD Projekt RED with Blizzard, anything they shit out is gold. Plus Hearthstone is everywhere and has a HUGE marketing campaign. Tons of mobile ads, youtube ads, etc. It's ok for them not to be on Steam because they are on a different league than many other developers.
But you have to see it from the point of view of a regular gamer, sure, he knows about Witcher 3 but how will he find out Gwent exists? He has to directly search for it. Most people will check steam, see it's not there and move on. Another advantage on being on Steam is people who DON'T know, they are just searching for random games, or looking at Free to play games... and they'll see gwent and give it a shot as it looks cool.
Steam right now has the biggest display in the world, and it doesn't cost an arm and a leg to be there, unlike what Blizzard does for Hearthstone. Basically being on steam is a very cheap customer acquisition plan.
As for Elder Scrolls it's not on steam yet, it's true. But there's still a chance. Also, there's another thing... it's shit anyway. So who cares