Well lets clear the definition of Surragate Mothers. They are women who let their womb be used to hold somebody else's child (also known as a test tube baby). The child when it is only a embryo lives in a test tube giving it the name of a 'test tube baby'. The embryo is usually made by the couple who hired the surrogate mother since their own womb is n/a to raise a child. The surrogate mother holds the couples child for 9 months until she gives birth.
People can adopt but their is of course something special wit your own blood your own child. And that is totally natural. Some don't want to go though the whoooleee process so they let the surrogate mother hold their child. The surrogate mothers are paid more then generously- around $30,000-$50,000 are some of the starting prices.