These days, Valve,s money comes from Steam. Their profits on that are stupid, like tens of millions per employee. Basically they just get to sit back and sell other people's stuff, and take a nice cut (30ish percent). As anyone who's ever had trouble with something will tell you, they have a minimal support staff, there's no phone number to call or anything, and responses take forever. Also when you really look at Steam it isn't that great. It isn't bad, but it is not some masterpiece of software engineering. Rather it was the first DD service to do a reasonable job, and thus it is what everyone started using and is in a nice positive feedback loop. Gamers buy on it because it has all the game and publishers sell on it because it has all the gamers.
Those massive profits let them do whatever the fuck they want in the rest of the company. They can goof off as much as they like, spend as much time as they like, release every game for free if they like, Steam makes WAY more money than they need.
That is also why Valve is so worried about the MS store in Windows. Supposing MS does a competent job of it (which at this point seems very unlikely), it could take away their golden goose. If people decided to start buying from the Windows store instead, because it came with the system and was integrated (like the Play Store on Android or the like) they could see their sales market evaporate and that would leave them in a rather uncomfortable situation. Hence the look at expanding Steam to other platforms, and the Steambox. They didn't just suddenly decide this was a good idea, they decided it when it looked like there might be a threat to Steam. Now given the utter hash MS is making of things, I don't think they need to worry, but that is the reason.
Also going back to the start of Steam, again you are correct. People seem to forget that Steam was hated, maligned, when it came out. The reason people used it was because they wanted to play Halflife 2, and that required Steam. They weren't pleased with it, but they wanted HL2. In that way, Valve got Steam to a large market. From there they worked on improving it, offering more games, etc, etc until eventually it is the juggernaut we see today.