As a user, I have to pay for iCloud storage for apps that use it. There is a free tier, sure... which most users will fill very quickly just with device backups and photos alone.
I don't recall ever playing for cloud storage on Steam, though.
CloudKit uses the user’s iCloud storage for private containers and bills the developers (although it’s usually free) for "public" CloudKit containers[1].
Steam Cloud is truly a backup service. It's not fast even for tiny amounts of data. They'll even kick you over to an even slower lane if you store anything over 250 MB.
Meanwhile, you can do near real-time app synchronization over iCloud between devices.
But yeah, it'd be great if Apple bumped up the free tier size. That said, I've never actually had any problems storing app data on iCloud. Apple users seem to either pay for more storage or not backup to iCloud, so from a developer perspective, eh.
You mean like CloudKit?
Apple SDKs exist and do things - including everything Steamworks does and quite a bit more.
If Apple decided to only allow apps distributed through them to use their SDKs or services, then would it would be fine because they'd be like Steam?