Because running multiple versions of a SaaS app is prohibitively complex.

A way to get both the advantages of SaaS and desktop + mobile apps is yet to be invented and popularized to my knowledge. The only architecture approaching this that I'm aware of is Urbit.

I must not have been clear. I don’t want a SaaS app, just a native app and local files. Charge for the mobile app too, and let me handle the cloud storage with either iCloud or google drive or something else