They probably don't because it would introduce technical debt. They want to support multiple platforms with one code base, and this is one way to do it. That being said, there are other alternatives...

The reason behind not wanting to have this debt points to deeper issues at Valve. They're a multi billion dollar company, but act like some local two location mom and pop store. The 350 employees they have is simply not enough to do what we expect of them.

They should do better, but they probably can't with the human resources they have. They have financials to expand, but I question what would become of them if they do. Right now and for the past 10+ years, its been status quo with slow changes and improvements.