One could certainly put feedback around the Crack circuit and get the output impedance super low, but it does indeed come with some negative side effects. As Gwyn pointed out above, there are many objective benefits of global negative feedback, and it is present in nearly all currently manufactured audio components.
Back in the day of vacuum tube amplifiers, the output transformer put a reasonable limit on the amount of GNFB that could be used before instability occurred, but with solid state amplifiers there is not such a hard limit. Manufacturers learned that saying your amplifier had 0.001% THD at rated power vs. your competition's 0.01% THD drove sales, so this trend continued until enough subjective listening tests were done to determine that minuscule decreases in THD didn't always correlate to a better sounding amplifier. Due to the subjective nature of this research, it took a few decades to be taken seriously, and that is about where we are now.