Marioface5: "Our BizDev and QA teams thoroughly research and evaluate every incoming title"
This doesn't really help us understand the process better, because we still have a lot of inconsistencies with what's on the store and what's rejected. To be honest, I don't even believe this claim. Cook, Serve, Delicious! was rejected for being "too casual", but most people who have spent a decent amount of time on the game will tell you that's not true. Meanwhile, Surgeon Simulator is up for sale. Aside from inconsistencies like that, there's still the issue of games that are very well-received elsewhere getting rejected or ignored here, such as many games mentioned in this thread.
My big takeaway from this and the general "Why isn't GOG releasing [game]" issue is that whoever they employ to evaluate games for release probably just have questionable taste. That's really not meant to sound mean, and I'm sure there are other, more business-oriented considerations that play into things (e.g., price points), but to me it's just the simplest and most obvious explanation.