Would have through incremental backup (replication) would be a core feature of most databases.

I think the problem is that replication alone isn't enough. You also need to support snapshots for backups because you don't want to replay years worth of replication logs to do a restore.

And when you support snapshots in your backup software, it's very tempting to then drop support for replication...

Replication logs also have another problem - since they include every data change, certain types of database updates, for example a large field that changes hundreds of times per second, can work out far less bandwidth to just snapshot periodically.

Overall, all this stuff is a lot of complexity most businesses don't want to have. They just want to have a simple backup solution and get back to building their product. And that usually involves using some managed database where backups are just an option in the control panel. Or a self-managed database where backups are disk snapshots. Or a script that copies a database dump to S3 every day. But it hardly ever involves streaming change logs...

I use borg backup for this very use case.

Not free, certainly. But it's a system requirement there to be able to rebuild the system in a non AWS environment.