Because there are more privacy guarantees there, depending on the provider. "But what if they violate their contract!" is some pretty tin-foil hat stuff.
How is this any different than a business running their website out of the cloud, assuming you are using a provider with appropriate contractual terms?
You can care about tracking and ads but still be comfortable storing your backups in the cloud, and many have been for quite awhile, even sometimes without encryption - that is totally different than e.g. Meta actively trying to track you and understand your relationship graph and your purchases etc.
But what if they violate their contract!" is some pretty tin-foil hat stuff.
The foundation of these businesses is stealing IP in bulk.So don't use them for inference.