How much do you make?

For most people, discussing money can be a root cause of drama, so it's easier to keep their mouths shut. It's also a tradition that exists in some cultures more than others. After all, a lot of us were taught not to discuss money, politics, and religion in polite company.

A lot of people's self worth is in part tied to their material possessions and income. People don't want to risk being seen as poor or less-than their coworkers, friends, etc, so they keep their mouths shut regarding income and attempt to show off (or fake) wealth through shiny new toys and bigger houses. On the other hand, well paid people want to keep their mouths shut because it can not only alienate them from their friends and family (who may well be making less and so be jealous), but it can also pressure them to be a low interest, no-criteria lender of first resort for people that know they have money.

I highly doubt it's been a trick (at least in recent times) to underpay people because it's so easy to find out what other people in the area in similar jobs are being paid.