Why is this?

Most corporations have the inertia of a rock when it comes to changes. There is no real reason behind it. Implementing IPv6 is a no-brainer. Same with NATing legacy systems so they can continue using IPv4.

Instead of manually juggling IP addresses, properly set up environments use hostnames and handle DNS automatically.

It’s basically “we/I don’t see any short-term benefit from it, it causes some setup work, and it is not important because everything still works”.