They bought hardware that doesn't even compare. There's a reason the iMac Pro went with the actual professional grade hardware, because that's their target. Using consumer grade and saying "look they're similar" is misleading and not a valid comparison.

Yes, the iMac Pro costs a lot of money. But if you buy the same components you're going to end up with a desktop that costs a similar amount of money, and doesn't officially run MacOS.

Of course not everyone needs the power of the iMac Pro, and most people lusting over it would probably be fine with the standard iMac. A better comparison would be with the standard iMac.

Configuring the 27" iMac with as similar specs as I can ends up being $3700. It's more expensive, the graphics card is nowhere near as good, and the CPU is a little faster, but only has 4 cores. CPU with similar speeds is -$200. It's still more expensive, but it comes much closer to the specs you chose to build the "iHac Pro" on, and uses a similar same grade of hardware.

You can also handwave away some of that price difference on the included keyboard and mouse, which your build doesn't include.