Because a few things could happen to the baby that the brithmother would be concerned about. What if the mother wanted to keep the baby, but doesn't have the time or money to take care of it? That can be a problem. Or if that it was an accident and they don't want to do an abortion, it may or may not have been there fault. If the brithmother was raped and founf out that she was pregnant it can be hard for her. What if the baby wanted to meet it's real mother when it is older? It can cause stress and anxiety upon both the mother and child. It may be hard for the birth mother to tell her child that it was created by rape. It also may be harder for the child to accept it and understand it. But if it is not a rape case, then it can just be hard to give up the baby in general because after awhile it mother starts to gain a bond and it is a part of them. It IS their child but they may have alot the need to go before they are ready to keep the baby. Plus, they have alot to think about along the way like the law, different adoption aencies, what's best for the kid amd also trying to keep calm as they go through this process of choosing and deciding if it the right thing to do. Especially to the adoptive parents that's always the hardest for them is choosing which family is ready to take care and love this child. To show two different kinds of love.
I think that it is equally harder/difficult on both of them because on the parents part they will probably never see their child again and they could not likely get the child back and maybe never get in contact with them. And the child will not have a parent and never know them and etc.
The age of the child being given up is one of the main points of this, of course the parent hurts and all giving up their child, but It's the child who will never see its parents again, and it's their life that is forever changed by this. I'm not saying that parents aren't affected, even if the parent doesn't care about its child, they are still affected by it somehow. Here is where the age comes in play, an infant is able to change its lifestyle and surroundings so much easier than a toddler, or an older child, because they are old enough to have memories of their parents and their lives before they were given away, it will always be with them whatever the reason, whether they weren't wanted, the parents couldn't take care of them, family problems, etc... But children never want to be taken away or left, they want to be loved, even if their parents don't love them.