In my case, the iPhone users around me bump into barriers that wouldn't be there if iOS didn't keep all data in inaccessible silos, but they don't see them as such. For example, someone wanted to back up all their iMessage conversations, including photos. This turned out to be a pain precisely because I couldn't just copy an "all iMessage data" folder like I could on macOS, but they didn't attribute it to the lack of filesystem access. To them (and most users presumably), it was just a problem of "why doesn't the iPhone let me back up iMessage to my computer easily?".
Does the local backup with iTunes not include iMessage?
I don't know too may users who care only about a specific aspect of their phone when backing up. I do have non-technical friends who do iTunes/local backups because they "don't trust the cloud", but I've never encountered a person trying to do this piecemeal.
For what it's worth, there are decent 3rd party tools like iMazing that are easy to use and do give you more granular control.
