Backups means you have history snapshotted in time.

Nextcloud does have done simple file versioning and a "trash" for deleted files, so you could get some simple protection from mistaken deleted files. But it's more "best effort" than "designed for backups".

Because it's a file sync program, not backup. There's a huge difference between the two.

Backups are a snapshot of your files at a specific point in time, generally backups will be done incrementally and then compressed and deduplicated, so multiple versions of a file don't take up massive amounts of space. Once a backup is taken it isn't modified again, so you can just roll back to a point in time and restore your files exactly as they were.

Plus, Nextcloud has had at least one bug in the past that corrupted all the files stored in it, so if you didn't have a real actual backup in place you were completely screwed because all the versions were corrupted too.

I had some files which rot away over the years, who knows which update borked them, i use it as a cache for real backups for important files