Because there's the example of the iPod. It wasn't the music player to have until Apple employees wore down Steve Jobs so they could let it connect with Windows.
Except that Apple still owned the ecosystem. Allowing you to connect iTunes on Windows is just like having Kindle on everything - a means to spread your ecosystem and platform. In fact, this is even BETTER for them, because you still had to buy an iPod!
The more you can treat the underlying OS as "dumb pipes", the more valuable/easily spreadable your ecosystem is.
Because there's the example of the iPod. It wasn't the music player to have until Apple employees wore down Steve Jobs so they could let it connect with Windows.