This is partly because Android apps may store data in one of several locations.

In previous versions of Android it was possible for apps to store data in an arbitrary folder on the device (if the user granted the relevant permission).

That resulted in it being quite difficult for the OS to identify which files and folders related to any single app.

In more modern versions of Android, most apps are no longer allowed to write wherever they want, and are encouraged to only store data in a private folder (like iOS).

A backup system is now present that can back things up in a manner to iOS, but it's limited to 25MB per app, and some apps opt out:

https://developer.android.com/identity/data/autobackup

What’s the motivation for app makers to completely opt out?

I understand that some data is considered cached and shouldn’t be backed up. But Android already allows that.

> Auto Backup excludes files in directories returned by getCacheDir(), getCodeCacheDir(), and getNoBackupFilesDir(). The files saved in these locations are needed only temporarily and are intentionally excluded from backup operations.

Overall though, this has been a solved problem since 2010 on iOS. True you only get 5GB of iCloud storage by default. But you can pay for more or use iTunes for local backup

So if one app writes files somewhere random, can another app simply read them? So two apps can share data that isn't something standard like the camera roll.

Even then, I don't see why the PC can't back up the whole phone.