You can consider me conspiracy theorist, but for me the whole story about WP/Nokia was to destroy the biggest non US tech giant, so the remaining are all US based. No one are now even close.
They've tried to build their own platform multiple times (Tizen the latest one), but they're still always stuck using Android.
They make a ton of money with their phones and making hardware that goes into the other phones, but that's nothing compared to taking a 30% cut of every digital sale on mobile.