That's not at all common sense. People that use multiple devices may want to put everything in the cloud service so as to access the same version on all devices.
Syncthing is also a good option but people have mixed experiences with that.
Granted, I’m using SyncThing between 3 different OS platforms.
It's besides the point: it's certainly not indicative of whether or not it's common sense that iCloud intends to sync documents.
I do pay for cloud storage to sync my reMarkable 2, mostly because they'll void my warranty if I don't.
Those people may learn the hard way that deleting documents on one device syncs that deletion to other devices in the same way that making changes to a document on one device syncs the changes to other devices.