Please note that the following is purely conjecture, but I do think it explains a lot.
I back Jim Sterling on Patreon. I enjoy his Best of Steam Greenlight Trailers series and hope that none of my upcoming projects ever appear on that particular series, I do happen to agree that ever since the floodgates opened with Steam Greenlight, the amount of crap that has gone onto the service has increased at an alarming rate.
Mind you, it wasn't always like this. Back when I was working on "SiN Episodes: Emergence," Valve had a severe interest in curating games that could appear on the service. Now, though, unless a game is likely to be rated AO or be considered pornography in certain markets, pretty much anything can get on Steam.
What changed? It's not the surfacing of first-party launchers like Origin or UPlay. It's not the resurgence of the indie game developer. It's not the rise and/or fall of indie portals. I think what changed is that Valve started to fear antitrust litigation. Let me explain.
Valve both owns Steam, the number one portal for computer game software digital distribution, and develops it own games. For better or worse, they are the major marketplace. If you aren't on Steam, you aren't visible in the eyes of most digital download consumers.
Back when Steam was only Valve's software, a case could be made that nobody else had a right to be on the marketplace. Even with a few select titles on Steam, Valve could easily say that this was part of a publishing deal with Valve, and therefore they weren't picking winners and losers. Up until they started allowing other publisher catalogs to appear on the service, it was still a "Valve is publishing these titles, therefore it's still a Valve product in a sense" service and could be defended as such.
The moment other publishers could put stuff on Steam, though, everything changed. At that point, Steam was no longer a publishing platform, but a storefront with negligible distribution costs and virtually unlimited shelf space...and they not only controlled who got what position on the shelves, but they also were stocking their own product there as well so they could be seen as having a conflict of interest.
Because Steam is now a storefront as well as a developer, Valve can't be seen to be picking winners and losers. If they say that product X can't be on Steam, if another game with a similar theme or quality bar gets on the service, the developer of product X could potentially sue.
Valve created Steam Greenlight specifically to get the "curation" part of the marketplace out of their immediate control. The assumption was that people visiting Greenlight would take their responsibility seriously and so Valve could allow the best submissions onto the service and still have some level of quality control on the service. Unfortunately, it's fairly obvious that Greenlight has been gamed to no end and the floodgates of shovelware and asset flips have been opened onto the service.
There are algorithmic solutions to some of these problems. Rating games that you own on Steam will do a good job of helping to quickly bury crap games. Using Steam refunds for absolutely horrible games will also help, because refunds also cost Valve a small amount of money in transaction fees and I'm sure that Valve factors refunds into how aggressively they shift products out to search purgatory.
However, there's only one thing that will guarantee that Valve is able to effectively run Steam without running into lawsuit bait as part of curation: spinning Steam off from Valve. The moment Steam is independent, Steam can easily start acting like a storefront only and not have to worry about conflict of interest.
Is it going to happen? Probably not. Hopefully Valve is able to find a way to more effectively curate their storefront without conflict of interests being shoved to the forefront...but I'm not holding my breath.