Realized some adjunct professors are not hired to teach but to advise

I just realized that all of the adjunct professors at my university aren’t hired to teach, but they’re hired to mentor since my department admits many master’s and doctoral students. They make so much money from these master’s students. Technically, these adjunct professors barely get paid but have to mentor 3-4 new students every year on top of their existing students. I found this to be so wrong. The tenured professors can’t mentor because there’s so little of them, and they wouldn’t have time for everyone.

What is going on with the system? Why can’t they provide some kind of security for these adjunct professors?