Why is my mobile hotspot limit so low only 5gb when I have a 50gb data cap?

The agreement is for your phone specifically, not your phone and everything else you have.  Data isn't unlimited if you want to think about it as you only get 30 days instead of the rest of your life.  There are restrictions.  

Best way to put it.  You own a lake and someone pays for their horse to drink from it, but then that person brings their pigs, cows, and other animals not their horse which breaks the agreement you had. You'd find ways to stop that or terminate the agreement.  That's how to think unlimited data on cellular networks are.