In 2017, 7126 new games was added to Steam. That's 593 games per month and 148 games per week, about.
In May this month alone, 768 was added.
Valve's organisation structure isn't ridged, it's fluid with members moving around to different teams whenever they want and doing whatever they want, as long the job is getting done, this is one of the reasons why people say that the customer service at Valve is really bad, because they don't have a customer service, it's staffed by them all, whenever they feel like answering some emails.
So, since they got no dedicated teams or jobs, you can understand how easy it is to add games to Steam without anyone there noticing because clearly they just not looking and if they looked, they might glanced at it's name and that's it, it's down to the whims of whoever is feeling like doing anything.
There is a "elitism" around Steam, I feel people feel that Steam should be reserved for AAA games only, but the reality is that it's a store front and Steam wants your money.
I personally, don't care about the trash games, I wouldn't even had known about this Active Shooter game if SJW's didn't complain about it, I don't have a problem with it too, even if it's in bad taste. pixels on a screen like fiction in books, don't offend me.
That said, am offended by Valve's and peoples hypocrisy as they get offended by games like this, but have 5000 hours in GTA or some other game.