The US is a pretty large market, plus carrier bundling is quite common in Europe too. You are usually free to choose a non-bundled phone as well, but I have no ideas what the numbers are.

Within the bundled packages, my impression is that Samsung is very dominant as well among the Android phones.

China and India are the two largest smartphone markets in the world, and I am thinking in these big markets carrier bundling isn't a thing at all.

New Zealand is outside the US and it's not unusual for people to buy their phones from their carrier alongside signing up to a plan especially if they are spending a lot of money. Phones purchased directly from the carrier are guaranteed to have the correct LTE bands which makes it easier to be sure you will get the best performance. Also carriers offer sweet interest-free deals with free stuff throw in which makes it more appealing than buying a phone outright. It's usually people buying cheaper phones that may just pair up an cheap phone from an independent store with a cheap plan often from a virtual operator.

It's the same in the UK as well.