Presumably because Red Hat is a business and is trying to make money. You only get updates if you have a subscription. If you aren't a subscriber, these images probably aren't what you want. Get the CentOS equivalents instead. If CentOS doesn't meet your needs but RHEL would, I can only think that you should be a paying customer then.
Thanks for the info, that's good to know. I was speaking form ignorance on that point, obviously (since I was wrong). The point still stands though, Red Hat needs to make money, so we shouldn't expect them to eliminate all their options for monetizing something (which I assume you agree with given your other comments :) )
So the complaint is that this Red Hat product is best for Red Hat's paying customers, and that if you want to use Red Hat's products without being a paying customer, you not only have to use the community edition's packages, but you may as well start with the community edition's base image? Am I understanding that right?
Because "FROM centos:8" can't exist until after "FROM ubi8" exists. UBI will offer betas for RHEL 9, 10, 11, etc. It solves a chicken and egg problem for partners who need early access.