They have other tools there to help as well. Microsoft is pretty good about helping devs here.
It's not like testing sites in Safari is better, Apple isn't even bothering to update Safari on Windows as it has been dead for nearly more than 2 years.
At least Microsoft is updating more often than Apple. They already have IE11 on Win10 dev VM there.
WebKit works on every major operating system, including Windows and Linux. IE/Spartan/Edge does not work outside of Windows.
There are some minor feature differences between WebKit, OS X Safari, and iOS Safari, but the reality remains that WebKit exists on Windows, can be built on Windows, and can be used on Windows, while IE/Spartan/Edge works on nothing but Windows.
Edit: I'd love to hear from the ethically bankrupt downvoters about which fact in this comment they are trying to hide from.
(And while I can't speak for the downvoters because I'm not one of them, I suspect that it's not the facts presented in the post that are attracting the downvotes so much as that it's written in the form of a flame.)
While we are at it, let's talk about Mozilla's browser and rendering engine that also works on every major operating system.
Chrome didn't render the same as Safari when Chrome used WebKit, either. You're caping up for this, and I can't for the life of me figure out why.
You can't say the same for Apple's Safari browser.