Because people get bored, and don’t realize that these days all of our devices are basically appliances.
People don’t want to have to relearn how to use their phone every couple years or so, and Apple knows this.
iOS 6 to 7 was a one time thing, the iOS team was completely restructured with A guy with pretty much the opposite design philosophies as the previous leader put in charge, so of course there was going to be a big change.
And even that change upset a lot of people, and it took a while before they got it just right. And now they have something that most people are familiar with and that most people are happy with, there’s absolutely no reason to change it.