Much of the chassis/power supply/motherboard expense is shared with something like the Z8, and some of the rest is functional for keeping the thing quiet (many or most of the intended users are video (or audio) pros who care intensely about that). It's not thousands of dollars of Jony Ive statuary - it's a few hundred dollars spent on cosmetics plus thousands spent on making it massively expandable.
Remember that a $3000 Z8 performs like a $500 computer - it's a 1.6 gHz quad-core with 8 GB of RAM, a hard drive and a nonfunctional GPU. Some of the difference between $3000 and $6000 is spent on the functional base configuration, so a $6000 Mac Pro performs like a $2000-$3000 computer, and you will have to upgrade some, but not necessarily all of the pieces (there is no upgradeable piece from the $3000 Z8 that is likely to be part of a functional Z8) . Some more goes to the power supply (the Z8 can have a power supply like the Mac Pro, but it's an upgrade) and cooling, and to a fancier motherboard with more built-in I/O.
I haven't used a Z8, so I'm not sure how loud they are, but I strongly suspect that it roars like a jet engine - I've used a bunch of non-Apple workstations, and most of them are built for engineering and they're LOUD. Apple spent a bunch of money avoiding that because they were aiming at the video market.
I tried configuring a Puget Systems workstation (they have a reputation for being quiet, and they are beloved in the video world) as close to the base Mac Pro as I could get. I couldn't get it exact, because they are not offering the new Xeon-W chips yet (my nearly equivalent machine uses the older, smaller socket 8-core Xeon-W 2145 from the iMac Pro). With the slightly less capable Xeon, quad-channel RAM (and a 512 GB limit), no Thunderbolt and a lesser motherboard, the Puget box came to just over $4800. Paying $1200 for macOS, Thunderbolt, the big socket and newer architecture, the fancy cooling (the Puget is in a stock Fractal Design case, and it'll get noisier under heavy load, especially if you add GPUs) and all the expansion capability doesn't seem way out of line.
Some people will choose one, some people the other, but it's not "the Mac's twice the price"...
Remember that a $3000 Z8 performs like a $500 computer - it's a 1.6 gHz quad-core with 8 GB of RAM, a hard drive and a nonfunctional GPU. Some of the difference between $3000 and $6000 is spent on the functional base configuration, so a $6000 Mac Pro performs like a $2000-$3000 computer, and you will have to upgrade some, but not necessarily all of the pieces (there is no upgradeable piece from the $3000 Z8 that is likely to be part of a functional Z8) . Some more goes to the power supply (the Z8 can have a power supply like the Mac Pro, but it's an upgrade) and cooling, and to a fancier motherboard with more built-in I/O.
I haven't used a Z8, so I'm not sure how loud they are, but I strongly suspect that it roars like a jet engine - I've used a bunch of non-Apple workstations, and most of them are built for engineering and they're LOUD. Apple spent a bunch of money avoiding that because they were aiming at the video market.
I tried configuring a Puget Systems workstation (they have a reputation for being quiet, and they are beloved in the video world) as close to the base Mac Pro as I could get. I couldn't get it exact, because they are not offering the new Xeon-W chips yet (my nearly equivalent machine uses the older, smaller socket 8-core Xeon-W 2145 from the iMac Pro). With the slightly less capable Xeon, quad-channel RAM (and a 512 GB limit), no Thunderbolt and a lesser motherboard, the Puget box came to just over $4800. Paying $1200 for macOS, Thunderbolt, the big socket and newer architecture, the fancy cooling (the Puget is in a stock Fractal Design case, and it'll get noisier under heavy load, especially if you add GPUs) and all the expansion capability doesn't seem way out of line.
Some people will choose one, some people the other, but it's not "the Mac's twice the price"...