Because it's better than supporting Microsoft's attempt to monopolise the Windows software ecosystem market with WinRT apps. Even Android allows you to use alternative app stores, yet Microsoft has complete control over the WinRT market and takes a cut of all apps sold. It seems that Microsoft has learnt nothing from the previous antitrust investigations against them.
The Windows Store is stifling innovation, whereas the Chrome Store is not. If Microsoft wanted to do make a closed ecosystem for a separate tablet OS then that's one thing; trying to force it upon existing Windows users is another (force being a relative term, as although they can't force you to upgrade it becomes difficult to buy a system without it).
You can install desktop apps from anywhere you like. Developers and enterprises can side-load Windows 8 apps. Nobody makes money on app stores, the 20-30% cut keeps the store operating and covers credit card fees. Are you really arguing that developers were better off before app stores came around? That they'd make more money without it? I don't think you've been following the software industry :-)
The Store is a boon for developers and users. For developers it lets them easily get their apps into the hands of users (with just one click), manages transactions, and simplifies the update process. For users it gives them an easy way to discover tons of great apps, while hugely increasing their confidence that they won't/can't hose their system (and thus their willingness
to install things to try them out).
As far as competition, the Android devices I've used only allowed app installations from one place (unless rooted/jail broken/dev unlocked). The same is certainly true for all
iOS devices. Yet on Windows 8 you still have complete access to the same desktop environment and installation sources/mechanisms you did in Windows 7. I really don't understand the complaint.