Valve is extremely lazy, and Steam has allowed that. They have by far the majority of digital PC game sales, and most PC sales are digital these days. So they make tons of money doing very little work. This has allowed them to do what they really like doing: Faffing about with random projects, not worrying about any kind of deliverables.
Unfortunately, lacking any real competition, Steam has no reason to really get better. The only store I would say is actually better than Steam is GOG, but they have the problem of not using DRM, which means many publishers won't release on them.
Steam isn't likely to get much better unless it has to. If some other company makes a good games store, as in one that actually hosts and deals with distribution, not just one that sells Steam keys (as many do) and it starts to eat in to Steam's market, then maybe Valve will give a shit, get a bigger dev team, hire some CS people, etc. However until then, they'll continue to just mess around.
They aren't beholden to investors, they make so much money (like $40 million PER EMPLOYEE per year) they don't have to adhere to any kind of schedule, and they are the one and only place many will go to buy games. As such, they can have it be a big shitfest and it doesn't really matter.
Personally, I go out of my way to buy any game I can on GOG, rather than Steam, if said game is available on GOG. They actually curate their store, make sure minimum quality standards are met, make sure old titles run, and don't put out unfinished games (aka Early Access). However, there's a lot I can't get there, and I'm one of the few people who do. Most gamers have this "Steam is the only place to buy, Gabe is god, all hail Valve," attitude.