This is a somewhat recent figure I found. https://9to5mac.com/2017/01/11/ios-market-share-kantar/
http://fortune.com/2017/05/23/apple-iphone-gartner-market-sh...
But the essential point is that nobody can afford to ignore Android and building to iOS-specific APIs doesn't make sense.
The market within the iOS space is huge and if building an iOS only app using ARKit is more profitable then why not ? It's all about ROI. There isn't an equivalent for ARKit on Android and building one would be costly and time consuming.
For example ARToolKit is available for Unity, or even Anddroid.
https://artoolkit.org/documentation/doku.php?id=6_Unity:unit...
Unless you are targeting a specific audience (for example artists that use the iPad Pro) for most developers it really doesn't make sense to confine yourself to iOS.
There have been smartphones for years. Apple is pretty late to the game.
For broad market maybe, but for niche markets a good example would be ForeFlight https://www.foreflight.com/products/foreflight-mobile/ It's pretty much the gold standard among pilots, from private to airline transport, and it's IOS only. Makes support and dealing with fragmentation a whole lot easier.
Surely you are referring to some future point in time, because most can still afford to ignore Android, use iOS-specific APIs, and be profitable.
The whole point of these APIs is to let developers make a better apps on iOS so people want an iOS device. If they port any of them to android all they're doing is making it easier for people to leave their platform.
Yes, you lose out on android for now, but in the mean time you support millions of iOS devices in a category that will be getting a ton of media attention in the next year.
Are they though? There's still a huge amount of people buying phones and computers, which cost a lot of money, periodically. There's paid services, sure, like Apple Music, which might become self-sustaining businesses, but a lot of these other "services" by the big tech companies are operated at a loss, like email, maps, cloud-storage, messaging, vide-calling, AI assistants, etc.
The only reason to make free services like all those at a loss is to bring value to a platform so users will stick to it, and Apple keeping its services platform-specific makes its platform even more sticky. Apple benefits from this directly, because it makes money selling its platform. To use iOS you need a device from which Apple makes loads of cash.
Google on the other hand is still essentially just an ads company with hobbies, and its profit from the platform its building is much less murky, indirect, and IMHO flimsy. The only way their platform brings them substantial profits is by collecting data to better target ads, which is concerning too.
The lifetime of devices is growing, so in the long term we have to see if Apple can keep selling as many devices as it does; their main source of income is not entirely future-proof. If you ask me, however, Google might be in a more delicate position, especially considering the growing issues with internet ads (concerning privacy, tracking, obnoxiousness etc) and that so many players are in the ads business nowadays. Some, like Facebook, are moving in deep-- challenging Google in user data analytics and engagement time.
tl;dr: Only some services (like Music) make money. All the rest are more valuable if they serve your core business, and Apple is very much doing that.
ARKit isn't a backend service it's an OS library so doesn't make sense to be cross platform.