100% agree

All replies focus on "service" or cloud-in-general is needed to implement sync between devices.

Why we don't have "own" sync-my-files as-service that can be connected to many apps?

I would prefer this one (in all incarnations - desktop/mobile/watch) to be just apps, but connected to something like "folder in dropbox" or "folder in gdrive" ...

My cloud my choice. Their program my data (here or in my cloud).

I know that it doesn't work in todays economy when everyone expects free in exchange for some invonvenience like ads/data-scraping by vendor.