For the same reason some people cling to their guns and bibles, many people cling to their iPhones thinking they are necessary for making video phone calls. While it is true that, because Apple has a locked architecture, Apple's Facetime App works only on iPhones. But there are many apps to make video calls between iPhones and Androids as long as the iPhone user has installed one of these apps. You can read all about it in the two cited references, but Facebook Messenger and Google's Dup are thought by many to be Facetime killers.
Link to Six Ways to Make iPhone/Android Video Calls
Link to Google's Facetime Killer: Duo