48V DC is mostly popular because human safety laws don't require special handling of 48v. Voltages above 48v/50v normally require testing/certification/etc.
48v is basically a voltage to avoid paperwork, while still being as high as possible to avoid spending too much money on copper.
I don't have a -48VDC power supply to test it with, but as far as I know +48VDC has the same effect. Firmly grabbing a lead with both hands results in absolutely nothing happening.
Doing it with wet sweaty hands is somewhere between "pretty painful" and "deadly". You're right that with dry non-sweaty hands, you can't even notice 48 volts.
48v is basically a voltage to avoid paperwork, while still being as high as possible to avoid spending too much money on copper.