They used to support Safari for Windows but it was a lot of work. It was worth it so that developers using Windows had a change of testing for Safari as well. Once the iphone came out and Macs became popular development machines in their own right, the burden of supporting Safari on Windows stopped mattering.

In Google's case the opposite applies: they don't have a development platform but need ubiquity because collecting user data is their reason for being. So they need to support a lot of platforms for the sake of their customers' (i.e. companies buying ads).