Stainless steel is a horrible heat transfer metal. Copper is right there on top as a great heat transfer metal. In fact, on space station, stainless steel was used as a heat insulator. I am confused on why someone would make a wort chiller out of SS.
Because it’s easy to clean, doesn’t exhibit the corrosion problems copper does, and works just fine in spite of being “horrible” at transferring heat. Mine will cool a 5 gallon batch to under 60 degrees in 20 minutes during the winter (45 degree ground water) and in about 30 minutes in the summer (ground water about 56 degrees) from near boiling temps. Good enough for me. Oh, yeah, SS is also a lot tougher than copper…
Because the thermal conductivity of the chiller metal often not a major factor in cooling time. If you could use the full thermal conductivity of copper your typical 25’ immersion chiller could a 5 gallon batch in about 50 seconds, give or take. Few people are able to acheive that… Instead you are limited by the flow rate and the conductivity of the cold water and the hot wort, which is about 30 times worse than stainless steel (and 1000 times worse than copper). Agitating helps to stir up the jacket of cold wort that forms around the chiller, but even so, the limiting factor is normally how fast you can move the liquid past the chiller, not the thermal conductivity of the chiller. It is also why counterflow chillers work well even though they have much less surface area than an immersion chiller.
It is also possible to make stainless steel tubing with thinner walls than copper, which makes up for some of the difference. I don’t know if the commercial SS chillers use thinner wall tubing than the copper, but my guess is yes, probably on the order of 1/2 to 1/3 of the wall thickness. That means the SS is only actually about 10x worse than copper. If you have a really effective whirlpool set up and very high flow rate, you can probably get to the point where you are limited by the stainless steel, but then you could just get a longer/larger chiller.
The main advantage of copper is actually that it is easy to cut, bend, and solder with simple tools. That matters a lot for people building them at home, not so much for commercial units.