Technically, Apple allows side-loading too. App developers do it every day, not only on their own devices but they can distribute apps outside the App Store to beta testers, journalists, etc. I've got beta versions of apps from half a dozen developers on my iPhone.

A few hoops need to be jumped through, but they're pretty minor - basically click "I understand the risk" a few times.

I suspect Japan wants side loading to be a smooth experience that works just as well as installing an app from the main Apple/Google stores. That's definitely not the case right now, users are heavily encouraged to only use the official store and there's a bunch of APIs/etc that cannot be used if you sideload.