- quality control for end-users. Like when Nintendo started with the NES.
- cutting the middle-man out of app marketing - good for third-party developers.
Microsoft does not care for neither party. Microsofts strategy with DOS/Windows was always about tight-knit contracts with OEMs.
And, Apple's strategy is also about a tight-knit contract with an OEM--itself. That's about as tightly-knit as it gets.
The fact is that me (and many thousand other) still prefer the reason (1) to the reason (2) and this is the success of AppStore.
Returning to "why Microsoft don't have it": because they don't need the indie developer and they will not upset the bigger brands to help a new market develops. The competitor of Apple in the mobile space are trying to start their store (Nokia, RIM)... if Apple can start the AppStore for OS X (and it have the same success, things I don't know if possible), we will, probably, see the Microsoft Store for Windows... otherwise I don't think so...
I guess this is why Google maps got a "go" despite using undocumented APIs the regular developers of the AppStore are not allowed to use?
It's not a as level playing field as you think.