> Why GOG refuses so many independent games???
Because so many independent games suck.
> And why Steam has no problem to release these games?
Because Steam isn't overly concerned with quality of games but rather the quantity of games and revenue generation. Steam has no desire to be a curator, offloading that responsibility to 3rd party "Curators" or to the individual consumer to decide. GOG has taken a different approach to be a self-curated store as an intentional core component of their different business model.
* My answers to these questions are just my blunt and honest opinion, and on certain reasonably well known publicized information about Steam and GOG's business models. Feel free to disagree with either Steam or GOG's choice of business models and decisions, or with my assessment of question #1 if desired, but opinions to questions like this vary, and well... you asked so there are my answers. :)
Pheace: Steam has gotten to a point where they can add as many games as they are now, something they simply weren't capable of doing till a recent system change a few years ago. (mentioned in one of the Steam Dev Days video's)
GOG is simply not capable of adding every game that comes to them which forces them to 'curate' their titles. And since it's a business that's going to come down to which games they deem would be the most profitable to them.
Yeah, not to mention that every single game in the GOG store is supported by GOG in a manner much more than the level of support one receives from Steam. Steam adding games is probably a mostly automated process with as little human involvement as possible and they're willing to ship just about anything if there is even a chance it will make money. Steam doesn't offer detailed personal 1 on 1 support to install, configure every game and fix problems one might encounter with the game, offloading that responsibility on to the customer and the publisher or developer of the game instead and washing their hands. They just sell the games like Walmart does, they don't tell you how to reconfigure your video card control panel to disable some setting to get a game they sold you to work - where GOG does do that.
GOG would have to hire about 10 times as many employees as Steam to sell all of the same games Steam sells simply because they would have to staff a shit tonne more support people than Steam does due to the level of support they provide.
Can you imagine that for 90% of the bottom barrel indie shovelware crap on Steam? They'd go out of business losing money doing that in a heartbeat.