I suspect most backup software on the computer pushes the backups to a network share somewhere that I suspect these ransomware packages go looking for and encrypt those files as well.
What if the backup system was remote and pulled the data from a network share on the client. If the client is infected, the infection cannot get to the backup file locations because they are not shared.
I realize this is not trivial for average users to setup, but I'm exploring this option for my home network. Setup NAS type server that looks for read only network shares accessable to userid BACKUP and slurps up any files it finds. Have it keep some kind of version control of a few days (multiple copies). Now when any new system is setup in my house (kid's laptops, wife's desktop, etc) I just have them create a read only share of their personal folders with a userid BACKUP and appropriate password.