Because I, an Apple user, would like to securely communicate with my Android-using family member. Security benefits both sides of the conversation. But since Apple doesn’t support it, I have to use a completely different service.
Because a lot of their mac users have android phones.
Same argument for windows...a lot of iOS users run windows.
Windows only cares about iOS and Android because windows phone died.
Apple has no incentive to actively support android hardware.
Services however makes sense and courting Android is a good strategy.
Because not everyone you talk to has an Apple device?