Photos aren’t something that is only inconvenient like movies to lose WHEN your drives die. They can be devastating because you will likely not get them back.
Backups are a lot of work to maintain, it’s not just simply copying the data to another array but also testing the backup regularly and making sure that they work and continue working. Backups are almost like a secondary job and many that self host are probably not interested in how much thought goes into it. Sure the web applications like Google Photos are annoying but they take away all of this that most people don’t think about.
The vast majority of people that read this will set up a server and have no backup. Many of them will eventually lose their data if they didn’t also keep a cloud service.