How about because it is hard to do.
I am a clinical research scientist as well and have a deep appreciation of biased ascertainment of endpoints. Nevertheless, it is fairly easy for me to hear differences in amps, preamps, dacs and CD transports that blinding won’t help cause I am impressed with how my brain tells me that my pre-listening biases are in the wrong direction. Great example, I put in an ASI Teknology modded Black Ice hybrid preamp in my reference system and was blown away by the spacious, clear, crystalline sound that had spot on tonality and tone colors. It replaced an Audiogon cult preamp and despite my clear expectation that the cult preamp would be better, it wasn’t.
If you are trying to discern very modest differences then blind testing is best. However, I can make a cogent case that if I am struggling to hear modest differences then it is probably not worth my time to try and figure out that puzzle - and certainly not worth much incremental money.
I doubt seriously if my preamp is the best around even to my ears and brain. However, I can guarantee u that I won’t be spending 20k to test the waters and find out. I have though been convinced that the modest bucks for Grover Huffmans Pharoah speaker cable and the nearly 2k per pair cost of the Zavfino Silver Dart interconnects is audible and worth opening up the purse strings - at least a bit.