Because I have to learn, understand what you wrote, probably more, and especially internalize its security implications. I currently understand all that for IPv4 and I’m confident I’m not leaving holes open when I self-host services. But of course it’s probably a good idea to learn and use IPv6. It’s just not free and when you have existing infrastructure and muscle memory on IPv4, there’s that much more work. If I was starting anew, I’d probably do it.