The $6k cost includes a ton of built-in cost for the fancy case, the server grade motherboard, and fancy power supply/cooling system.
As far as I'm aware, you're not even capable of building a Threadripper platform with that much RAM... because you're limited to 8 DIMM slots? There's a few things on the horizon that might address that (namely, 256GB DIMMs), but I'm not sure those are released yet... and I bet they cost a lot more than 128GB DIMMs.
Not saying it's not overpriced... but Dell et al. probably will gouge you just as much if you try to build a Xeon workstation.
Hopefully Threadripper drives down the Intel pricing even on the server side, cause as of right now it's just crazy.
If it is actually an issue of unit cost, sell a lower-end base. If it is to recoup development costs, find a way to price discriminate.
Otherwise, you are charging low end users for functionaility that they will literally never use.
Also, if you only NEED 32GB of RAM, then don't buy a Mac Pro. This is just like buying a Xeon motherboard anywhere else and only putting 16GB of RAM into it. Sure, you can do that, but it's a waste of money.
Intel is HP in the 2000s. They're not going to right themselves gracefully or intact.
Once you leave the consumer grade category, the prices go way up. Xeons, server motherboards and ECC are not cheap.
This is an actual pro level workstation for video and audio. It's not a consumer or even developer desktop. And relative to those markets, it's priced competitively. For example, a basic RED camera setup will run you at least $20-30k, easily.