Those actors usually exchange some of their up-front payment for a percentage of the film's profit. In other words, they are taking on some of the risk associated with film production and betting on their own appeal being enough to turn a profit and generate income.

It would be like a bricklayer not taking any payment until the house was sold and therefore taking on the risks associated with the housing market. Most probably wouldn't want that. You can't have your cake and eat it too.