Because Time Machine does versioned delta backups and, as far as I know, no cloud provider offers the ability to store that much data unless their business model revolves around backup solutions.
But it’s just for Apple users? A terrabyte per user is surely within Apples reach? I can backup my phone to iCloud, but not my MacBook. It’s a glaring hole in their end-to-end experience.
Macbook drives typically can go to 1TB or more. You'd need much more storage than that to properly do a versioned backup solution that was online and that doesn't include any of the other uses of that storage.
