Sounds logical, but it doesn't seem to be the case. The backup options are "Photos/Videos" "phone data" and "both". I don't think phone data includes all app-local data. Contacts, calendar entries, and such, get synced but that isn't due to a global backup process that is the Google apps syncing with your Google account. Other apps could do that with the right integrations, but not all have the option and either have no backup or backup by syncing to their own service or an external option like an S3 compatible store.

It is somewhat disjointed.

When I last changed phones, between phones from the same manufacturer both running recent Android versions, the "copy apps, settings, and data" process didn't include all app data either so I need to take extra steps.

I don't think there will be any big push to address the matter, because for the vat majority of users it isn't a big issue: most of their data is synched to various services anyway and that which isn't wouldn't be particularly missed if lost. There are very few app dealing with important data that are local-only.