When I google Thermidor built in fridge I see about 3500/4000 dollars
Where exactly are you seeing this ? I've been loosely associated with the appliance industry for 25 years and I'm pretty sure a new, full size Thermador refrigerator has ever cost $4000 or less.
American "built in" and integrated fridges are a bit different animal than their Euro counterparts. For decades, Euro fridges were concealed by putting them in a cabinet with four sides and using a cabinet door on a simple but special sliding bracket that is attached to the fridge door itself.
This works because "regular" euro fridges have been designed to function enclosed. Regular American units need lots of air around them to function and they are deeper than the cabinetry that surrounds them. They won't fit in a cabinet and if they did they'd bun up the compressors.
So, for the few people that wanted a concealed fridge, but didn't want to import a Euro unit and couldn't deal with the small size even if they did - SubZero developed the "fully integrated" 700 series in the 90's. These weren't regualr fridges or built-in fridges but something new. They had purpose built hinges and depths to conform to 24" cabinet carcass depths.
Now the Europeans have jumped in the game with new ground up platforms that are larger and more feature laden than the regular EU fridges they sell. Bosch, Gaggenau, Thermador and Miele all share a factory in Turkey that churns out these large fully integrated units largely for America but they sell in other markets too. These are more costly to make, sell in fewer numbers and have more cache. Which is really why they cost what they do - because the market will bear it.
Kinda like cars. India, China and Korea can all produce comfortable, functional and inexpensive cars. We're not clamoring to buy those over American, German and Japanese versions even though they cost 4x more like these fridges.