I'm not surprised. At all.
The way Valve is working allows it to do things others can't do.
Their workers have perhaps the most liberty in the industry in trying out and implementing their creative ideas in the games they make.
Instead of having a single person being the ultimate manager and lead designer of a game and be the only one who decides what gets in the game and what not,everyone who is working and has an idea is allowed to implement it,and shares it with other co-workers,and the mechanics/features etc are decided in democratic fashion -with individual workers voting if they like a feature or not.
Simply put,the combination of a different number of minds is more thoughtful and creative than a single person, which makes Valve's games to have something special.
Now their own developed games isn't the only profit bringers,Steam is another one.
Steam rules now digital game sales,but it wasn't always as popular. It exists like 10 years. When Valve made Steam it was alone,nobody else think of that and Valve was considered weird for doing it. But after lots of support in ways of extending the library,and adding features and convenience,Steam became the favored digital distribution channel for games for the majority of PC gamers and developers/publishers too.
Steam is now the most accessible digital distribution games store. Not only it has the largest library,but it's also supporting more input methods as you can use it with both mouse and gamepads,and it also supports the most paying options,like paysafecards,and its growing support list for various different aspects gets bigger and bigger.
While others have made Steam like programs recently,like EA with Origin and Ubisoft with Uplay, Steam holds a decade of experience and updates,and is at least a step forward in every aspect,meaning it leads the race of features and other ones try to catch up.