It's actually pretty simple: they have created the perfect conditions.

First, theirs is not a customer but a fanbase. People want to be part of the apple universe because they are fans of it.

Second, they have made it extremely difficult to get out once you enter their ecosystem.

So basically you have a good bunch of people who are fans of their products (so not buying because of functionality but because of fidelity), and another good bunch that are captives of a previous decision and can't get out easily.

There are other minor groups in their customer base, but they aren't that relevant to make a dent in their sales if Apple loses them.