Because Valve is perfect and the "savior" of PC gaming, didn't you know ?
All the while Steam prices are the worst you could find in almost all countries in the world compared to retail but that's ok because it's Valve, and it doesn't affect their mostly US based fanbase, you just have to look at their forums to see that the rest of their customers aren't as happy with them.
That and Valve is more of an objective 3rd party at this point. They have 1 release a year so other companies using the steam store to sell their products do not have to fear any favoritism. You can realistically say that Valve is as much developer as they are Digital Distributor at this point. EA is a mega publisher that would be the biggest in the world if it wasn't for the Activision/Blizzard merger and comes out with 2-3 titles a quarter.