Honestly, I can't think of a single app that doesn't back up its own data. Meta apps have their own backup mechanism, games all backup either through their own servers or Play Games service, pretty much everything I use is a service of some sort.
But yeah, I guess Google isn't backing up the state of the offline chess app I downloaded 10 years ago...
Not the same person, but this affects me and I can explain. I have around a hundred apps installed, and use them just frequently enough to keep them around. All of them can sync their data back to a new device without issue.
The problem is that when a new or wiped device installs those apps again, they won't have any of their account or configuration data. Even with a password manager to help, logging back in and configuring my preferences is a tedious, all-day affair. In years past, there were ways to transfer account data, but they stopped working a long time ago. I am eager to find a solution to this that works without root and on modern Android.
It's nice that you don't have this problem with the apps that you use, but others do.