For me it’s because it’s just a tool that enables me to do shit, and it doesn’t need to be anything more than that. Can I call/text, browse the internet, check my email, etc? Yes, okay good enough.
I don’t need my phone to be some sort of expression of my identity or whatever, just give me squares that do the things I want when I press them.
Whatever works. Often, simple and consistent wins with software/UI design so in some ways I can see why they keep it so basic.