There are technical reasons... Uploading data to Find My requires a connection to apple.com which is authentic (ie. Uses crypto keys from the apple device, and apple won't give those keys to an android manufacturer).
You really think Apple would say "no" if Samsung or Google said they want to connect their devices to the Find My network? They'd give the keys in a heartbeat.
But neither G nor S is willing to provide extra coverage for Apple nor do they want their phones connecting to a system they can't datamine.
I think the crux of the issue is that G & S won't work with A because A runs things like a dictatorship. Which is ironic considering how many international standards/specs A uses for their devices.
Pretty sure Apple would make a special "Apple-Fi" incompatible with standard Wifi if they could get away with it.
Currently it's a unique selling feature of the Apple ecosystem. If every phone could do it, then Apple would lose this benefit, and Apple would still be paying for all the servers.