Oh that's easy. It's because Microsoft does not care about their customers enough to listen to them and provide features they want! Instead, their developers are allowed to have a "we know best" attitude and force people to adapt or leave. I'm sure Microsoft thinks they are so big people won't leave. Unfortunately, they are somewhat correct given the huge investment companies have in Microsoft...it is hard to shift away. But as a consultant I've seen companies do just that when they finally got fed up enough and refused to play Microsoft's games anymore.
Microsoft has received numerous, legitimate complaints regarding this limitation. Instead of simply making their app available for Windows, they argue about why customers don't really need the feature. I even saw a Microsoft support forum where the Microsoft employees were trying to tell customers how to get around laws in countries that forbid forcing employees to use their personal cell phones for authentication.
You have a choice -- quit using Microsoft products, or fork over the money to adapt by purchasing additional hardware for employees who do not have / do not want to use a personal cell phone. I think we can safely conclude by now that Microsoft has no plans to listen to customers needs by providing a good non-cell phone option.
