Why is it taboo to talk about pay?

imagine you and your coworker complain about the way your boss underpays you. neither of you had any idea how much the other is making, and I just flat out ask "well how much do you get paid?"

you say "you can't ask that!" and explain to me that it may cause tension in the workplace. I point out that any tension should be pointed towards the person who is underpaying you. If I find out that X is getting paid more than me for the same position, i'll be more likely to ask for a raise rather than assume i'm getting paid a fair wage. shouldn't that be something all coworkers want to team up to do?