You're right, it's not fair. But a lot of women in Hollywood and other industries have been protesting for equal pay. In Brando's case, it was "star power." Movie stars like him are so big, they are almost guaranteed to sell millions in movie tickets. So the studio agrees to their outrageous salary because they will make hundreds of millions in profits if those actors star in their movie.
The same thing happens today with DiCapprio, harrison Ford, Clooney, Brad Bitt, Matt Damon, etc., etc., But also actresses get overpaid ridiculous amount of money because people go see movies just because those actresses or actors are in it. Not because it's a good movie.