It doesn't have to be difficult to self-host. Like another commenter said, in a diffe world that could be the default. iCloud gives apps an API to sync. The backend doesn't have to be a data center, it could be a time capsule in your living room. You could connect using a private wireguard lan. The protocol could even be built out to support redundant time capsules in case one failed.

But my parents wouldn't want to pay $500 for the hardware, and companies don't want to give up the monthly fees.