Why do people confront somebody when theyve found out the other person has said something nasty about them?

Some people find some sort of satisfaction [maybe not pleasure so much as like the relief you feel when scratching an itch] in confrontation and arguements.

They feel validation to their feeling of offense when they receive a reaction from the person they are confronting.

well, if it's lies and affecting the person's reputation, it's worth confronting the culprit.
I'm sure many people have lost opportunities due to jealous people muddying their name

So it's all out in the open and there's no fake crap going on behind your back. That's why. Why say things behind someone's back that you aren't willing to say to their faces?

That way you know what type of person they really are and you also find out if the person that told that were telling you they truth or were just starting trouble between the two of you