Less than 1% of all computer users regularly backup their data.
The main reason for this is people feeling that backup systems are too complex - and picking files and folders to backup is the typically the most complex part of the process. Thus, at Backblaze we focused on making it a zero-configuration setup by backing up all data with no questions. This speeds the process of getting setup and reduced the likelihood of forgetting to select something important for backup. It does backup a bit of unnecessary data, but this is typically minor in comparison to all the photos, music, movies, and documents that users actually do want backed up.
Our approach isn't ideal for everyone, but it is easier for 99% of people. (If you have a specific folder or drive with a ton of data you don't need it, you can exclude it. But if you just have a ton of small folders, let them backup and don't waste your time managing the backup!)