You've got it backwards. Valve wasn't always so wealthy and it doesn't explain why so many video games with 50 million dollar budgets are total crap. What does explain it somewhat are games like Duke Nukem which was bounced around from one developer to another for years, universally panned by critics and gamers alike, and still raked in the big bucks. As long as Publishers can continue to exploit developers and continue to exploit gamers for a fast buck there is no incentive to do otherwise. Gabe Newell and Valve represent one of the few companies in the business fighting that trend and that's part of the reason they are such a huge success.
Its the publishers in particular who need to pay attention. THQ recently received an anonymous letter from an employee lambasting their management for driving the company to ruin. EA has had a horrible reputation for years for exploiting developers and gamers alike. Steam and Valve games have forced them to not only start providing better quality games and service, but pay attention to just how badly they exploit their developers and customers. MS Games for Windows Live still sucks as a service. They need to get through their heads that its not all boom and bust and the demand for consistency and quality is enough to make billionaires.
What?
Valve lets people put almost anything on Steam, including things which won't work with newer hardware very well. There's no QC required for games which are put on Steam.
Compare it to something like GoG.com (a very different model, obviously), where older games are updated in order to play nicely with newer systems.
Valve doesn't force people to provide a better service at all, they provide a way to make more money by charging retail prices or more, and getting a higher margin.
When it comes to making games, they just buy in ideas and studios, let the people make the game, then wait to buy their next idea.
A model of creativity.
Sure, they might not push people to make games, which is why they don't make many games, but equally they don't have any outside shareholders to answer to who want money asap, which is the advantage of being a private company. It's an inherent problem with any public company that shareholders have demands which don't necessarily allow people to run things the way which might be nicest.
(Amusingly now that Turtle Rock (L4D) have split from Valve, their next game will be published by... THQ (if THQ lasts that long)).