Apple doesn't make a product that costs less than $430 (the SE). If you need a cheaper phone, you have to go Android. For lots of people, $400 is a lot of money (especially if you're also buying phones for your kids, who are likely to break whatever you buy them). In lots of families it's iphone for the grownups, android for the kids. And that's just within the US.
On a global scale, if you're selling phones outside of the US and European markets, some part of your product lineup has to be in the sub $200 range (in addition to the flagships you sell everywhere). For example, in India, Xiaomi sells the Redmi 9A for Rs 8,000 (about $100). They're competing with Samsung phones we don't see in the US, like the Galaxy M series, which are in the same price category. Those phones exist because (a) people need cell phones (especially in places where a phone might be your primary access to the internet) and (b) Apple is choosing not to go after those customers.
Even though Samsung makes a more expensive flagship, the average income of Android users will always be lower because (a) there are more poor people world-wide than rich people, and (b) Apple isn't making phones for poor people, while Samsung, Xiaomi, Oppo, etc. are. Apple's strategy is totally working for them (selling a luxury product at luxury prices works well) but it also means that most poor people (which is most people) won't ever have one, so the average income of an iOS user will always be higher (along with the other things that go along with a higher income, like higher spending).