Why are equipment racks so expensive?

I share your feeling re prices of equipment racks.I have three amplifiers, one requiring a preamp, three cd transports three dacs, turntable,and  three tuners.I tried rotating each system onto a four level vertical audio shelf as I wanted to hear it, but old age, bad back and laziness made me look for alternatives.Googled make your own audio racks and came across a style that allowed me to create the number of levels, height and depth and width exactly required to hold all of my systems at one time. Uses 1/2" threaded zinc plated rods. Big zinc plated nuts and washers on the threaded rods and 5/8" thick shelving from Lowes.All of the metal hardware was ordered from McMaster Carr online.Let my wife choose the paint for the shelves.Couldn't be happier.I am a total klutz when building things and this did not come out perfect, but I made it work. It looks good enough for me and is functionally exactly what I wanted.I only screwed up when trying to drill the eight holes through three shelves simultaneously, in that I did not have enough clamps, so I had to move them and two sets of holes  came out slightly off center.  I
cant see it, but I know its there.Unless you want to purchase a really beautiful expensive high end audio shelf that you can brag about, I highly recommend you take a chance on this style.

I use an Artesania rack and it's the centrepiece of my living room.
It is massive, beautiful and functional. Aesthetics are worth paying for.

Having spent many pandemic lockdown months meticulously putting together my headphone system, it became very apparent that I had to get my gear off of the dining room table. So instead of chasing less expensive solutions like those available at IKEA, I simply went to the hi-fi store where I bought a lot of my stuff, and looked at what they put their $100K worth of components on. Yes the initial sticker shock was quite the slap in the pants, but after reading extensively I decided that investing in a five shelf Solid Steel rack was going to be well worth it.

...with prices:
Single A/V Racks (audioadvisor.com)
expandable, upgradeable

While the Lack is a very good shelf for next to nothing, Finite Elemente and SRA racks (examples I own) are better sounding and better pieces of furniture. I am not sure of the cost question. One is designed to be ultra cheap and mass manufacturered. A smart guy named Ken Lyons promoted their use. FE and SRA are much better pieces of furniture, sound better and not mass produced.  

Also the Lack is a shelf, not a unit. Solidity and stability was not the point. They are very light and rigid to yield clean bass.  "Solid" actually means nothing to me by itself as far as performance is concerned. My flexy was solid and sounded horrible.