Apple created that demand. They were exclusively on AT&T at first, and aggressively negotiated that contract so they wouldn't have to have carriers' horrible bloatware. Then the exclusivity made negotiating with other carriers easier.

Google didn't even make any phones until much later. Instead they gave phone manufacturers the green light to do whatever they wanted. They ended up designing phones later because of how crappy the Android phone ecosystem had become. Google could have made Nexuses from the get-go.

Google had plenty of leverage early on with carriers that didn’t have the iPhone. Google still has leverage by threatening to withhold Google Play Services. The last thing carriers want is not to have an alternative to Apple and no cheap phones to entice customers.