Without labouring this too much, because they're employed by businesses who make money and don't care for the enthusiast value or sound. Why have the 37s survived where 20s/31s/33s/47s etc etc have not (Rhetorical question)?
20s presumably suffer from having to operate in pairs, having to work out how to turn a loco somewhere or eventually have to run nose-first. I think they enjoyed a long tail on their careers because DRS built their operations off the back of them (when they were cheap ex-BR cast offs). Likewise Harry Needles was working out how to turn 20s into remotely operated shunters that could creep along at 5 mph. Innovation off the back of a cheap platform.
I gather Network Rail got rid of the last of their 31s when the wheel sets were worn out and it made no economic sense to replace them. This is why they were cast off cheaply.
I assume 33s are dogged by the same corrosion issues that appear to plague 26s and 27s. The Sulzer-y bits are good, but the BRCW metalwork is dodgy. I was looking at a 26 under restoration at Bo'ness recently and there were patches of filler almost continuously above the sole bar.