Maybe it’s because you can spend less than $6000 and have a more powerful machine.
I see a couple of things here.
1. Support:
It is a very attractive proposal for users that will guarantee that its Afterburner/MPX Modules ( Or potentially other internal hardware ) will just work and not have to configure and troubleshoot most configurations. (Unlike PC Alternatives) With additional third party support with MPX modules such as AVID for ProTools and Promise hard drive support will make it even better.
2. Custom software/MacOS drivers
More powerful is relative. While price/performance hasn't always been Apples for-tay, Apple tends to squeeze out more performance with less powerful hardware which often out performs faster alternatives. Most likely due to custom drivers/Applications/MacOS for a limited set of hardware. With the addition of Third party support for 14 different companies for products like DaVinci Resolve, Red cameras Redcode/R3D/Red Raw, Atomos, additional Metal 2 support for applications and/or codecs.
Unfortunately the new Mac Pro system is now beyond my own price point and see me moving to PC alternatives. I would be wonderful to see a prosumer version at a lower price which I'm doubtful.
Sometimes you have to be careful for what you wish. Users wanted a more powerful/upgradable/expandable system. Well, we got it, at the expense of pushing out prosumers. But these systems are not for them anyway. They will probably point to their other systems: iMac Pro, MacBook pro ect.