Answer: Because they don't know the words (rim shot).
We all know about humming single coils and how middle positions which combine pickups cancels that hum. But my Tele has a special hum, above and beyond that, that is only cancelled by touching some metal part of the guitar. It drives me nuts but there doesn't seem to be anything that can be done. The goofy part is that it only hums in church and in the room where I teach, both of which have great big fluorescent lights. My Strat does not hum in either place. Everywhere else is just fine. The repair guy I use (who is very good) says he could reduce it a little, but not by much.
I guess some guitars just hum!