I don't think most iPhone users DO know what version they have. But they update pretty fast.
Apple pushes users to update with notifications on the device. They add things people really want (there was an article last year or so that the releases with new Emoji get adopted much faster, iOS stickers caused a big push). Plus you get the network effect of someone telling their friends there is a new version. If none of that works, apps requiring new versions of the OS push some people.
Android manufactures may not make new versions for old phones, may not push the notifications about them, and there are lots more kinds of Android phones so just because Jim got the new version doesn't mean Barb can because she has a different OEM. Plus because so many Android devices ship with old software apps are often designed to run on very old releases so they wouldn't push people to update nearly as much (and that's if they can update, you can't be sure a new version is even available for your customers).
This problem is a LOT harder the way the Android ecosystem is setup.
