I was a professional, and I didn't have a backup of my personal system for about 2 decades. I just didn't have another 4TiB of storage to copy my media library onto. I'm now on backblaze, but there was a long time there when I did not have a backup even tho I knew better.

Also, even in a professional setting, I've seen plenty of "production support" systems that didn't have a backup because they grew ad-hoc, weren't the "core business", and no one both recognized and spoke up about how important they were until after some outage. There's virtually never a test-restore schedule with such systems, so the backups are always somewhat suspect anyway.

It's very easy to find you (or your organization) without a backup, even if you "know better".