Because they didn't make it knowingly. They're lacking knowledge of good and evil, remember?
You are not responsible for sins you commit without knowing it is a sin. For example adam and eve were naked but they didnt know and it wasnt held against them.
Adam and eve did know not to eat from the tree of life and the tree of knowledge of good and evil. They were told multiple times not to eat it and they were told the consequences multiple times.
They only knew good before eating from the tree and because they only knew good they didnt know it was good because they had nothing to compare it to. Havent you ever heard that you dont know what you got until its gone? Well you dont know what good is until you know what bad is.
When they ate from the tree they didnt learn what good was they already had experienced that. They learned what bad is because they experienced what being bad is and they knew the difference between good and bad. They already knew the consequences but it wasnt until they ate the fruit that they experienced what being bad is.
They were told what the concept of bad and good were. But there is a difference between hearing a concept and exeriencing it yourself(knowing it).
I can tell you if you put your hand in fire its bad,it hurts and your skin will blister etc. At that point you will understand the concept of what happens when you stick your hand in fire. But you wont know what it feels like, you wont know what the blisters look like and you wont even know if i am telling you the truth. When you stick your hand in fire then you know. But am i to blame because you stuck your hand in fire even though i told you everything that would happen to you? Do you really have to stick your hand in fire to understand the concept that sticking your hand in fire is bad?