Besides the reasons others have already mentioned: you quickly train yourself not to pay attention to anyone trying to attract it in public, especially on the metro. Otherwise you're inviting inconvenience as they try to follow up and you (if you're nice) try to disengage politely, if not inviting actual trouble or harassment. This is not a good environment for paying attention to art, recognizing celebrities out of context, human connection in general, etc.
Anyway, my personal reaction to a good busker is "well, they're still making noise in public, but at least they don't suck".
Anyway, my personal reaction to a good busker is "well, they're still making noise in public, but at least they don't suck".