Because there's nothing on the latest versions of macOS and iOS worth updating for at least for me. So if I do install them I'll just have slower systems due to the extra bloat and apps from Apple that don't work as well.
I'm avoiding moving onto iTunes 12.7 because they took out the apps in an incredibly stupid move. I have iOS apps installed on my iPhone and iPad so I'll be downloading the app twice instead of once to my Mac and syncing it to my devices. So there's no way I'm going to High Sierra while iTunes is so fucked.
And the Music app keeps getting more screwed up with each release. I haven't played around with iOS 11 but iOS keeps getting worse. 9 to 10 was terrible for handling notifications on the lock screen. They added a step or two in order to mark a reminder as completed. Just a bunch of little things like that which make the experience worse and I hate to see what they've done in 11. But since there's nothing that they've added that interests me all that it will do is slow down my devices.
I'd love for them to just put a patch out for Sierra and iOS 10.