The main reason IMO to block rooting is to stop resellers selling phones with preinstalled malware. If the phone has two Amazon/Aliexpress sellers, you're going to pick the cheaper one right? With who-knows-what alterations? It's a really prevalent problem and most people are not going to notice the "insecure" warning at bootup.

Phones can and do have a warning that they were rooted on boot. So this is not an excuse. But don't worry, I'm sure there are several marketing teams at work on new excuses why your computers should be controlled by benevolent corporations and not you.