I suspect the biggest reason people stick with a platform is simple lock-in. I am on iOS because it has an amazing app ecosystem and tons of great games. For the first time since the 90s I am buying software again, and switching platforms would render that "investment" useless.
I think more people go from Android to iOS, mostly based on economics. iOS users spend more, so in theory lock-in impacts Android user's decision to switch less.
So in short: Apple could make an Android phone, but there is really no point. It would dilute brand identity and encourage the loss of App Store revenue. The Android users who don't buy apps are the least likely to lock-in, and core Android people wouldn't switch anyway.
Remember: Apple doesn't need market share, it's after market profits. An iOS user generates far more revenue.