It's easy to understand that Apple won't develop their applications with special features on other platforms.
But why did Apple make iTunes and Safari on the Windows systems? The answer is that there are still many people using iOS devices with their PCs and they have to find a way to make there iOS devices easier to use, and then Apple released iTunes for Windows. Apple has already stopped updating Safari for Windows now because of iCloud. So Safari 6 is only for OS X Mountain Lion.
GarageBand is one part of iLife which makes your life with Mac more colorful. If you love it and are familiar enough, you can easily update to Logic and begin your music life with Mac.
And that's what Apple wants.