Because you are not listening blind. If you did demo blind, you’ll find a $500 DAC and a $2500 DAC (assuming cream of the crop for their price points) are not going to sound dissimilar.
If an amp or preamp has good linearity, low distortion (THD+IMD), high SNR, high channel separation, etc., then it’s hard for them to sound appreciably different.
If an amp or preamp has good linearity, low distortion (THD+IMD), high SNR, high channel separation, etc., then it’s hard for them to sound appreciably different.