Eating the forbidden fruit altered humanity's makeup.
When Eve and Adam ate the forbidden fruit from the tree of the knowledge of good and evil in the Garden of Eden, it changed them. That sin altered their design.
First, before they sinned, Adam and Eve had less awareness. After they sinned, they suddenly realized they were naked.
6 ... [Eve] also gave it to her husband with her, and he ate. 7 Then the eyes of the two were opened, and they knew they were naked. So they sewed fig leaves together and made themselves coverings.
(Genesis 3:6b-7 OSB)
The realization of nakedness produced within them feelings of shame. Shame was new. Before they sinned, there was no cause for shame. Sin introduces shame, that's true, but this specific shame was a desire to avoid embarrassment. They didn't want to be seen naked.
GOD did not design man to know shame or embarrassment. But eating this fruit changed humanity's design. Suddenly, man knew things that (s)he didn't care about before.
Having disobeyed GOD's command (sinned), combined with their new heightened awareness and shame, Adam and Eve felt the urge to hide.
8 Then they heard the voice of the Lord God walking in the garden that afternoon, and Adam and his wife hid themselves within the tree in the middle of the garden from the presence of the Lord God. 9 So the Lord God called Adam and said to him, “Adam, where are you?” 10 He replied, “I heard Your voice as You were walking in the garden, and I was afraid because I was naked; so I hid myself.” 11 Thus He said, “Who said you were naked? Have you eaten from the one tree from which I commanded you not to eat?” 12 Then Adam said, “The woman You gave me, gave me of the tree, and I ate.”
(Genesis 3:8-12 OSB)
Adam seems to have not understood this urge to hide from GOD. Adam's explanation was that he hid because he was naked. But we know from our own experience that humans hide when they feel guilt.
Adam knew what had said about not eating from the tree of knowledge. And Adam knew that he and Eve had disobeyed. Adam knew they were guilty. I believe Adam used his nakedness as a coverup for the real reason he hid from GOD—sin had made him unholy.
But, sadly, human sin didn't stop there. No, every single person to live committed sin, one after the other. The Bible says that Adam brought sin into the world and, because of this, all sin.
12 Therefore, just as through one man sin entered into the world, and death through sin, and so death spread to all mankind, because all sinned— 13 for until the Law sin was in the world, but sin is not counted against anyone when there is no law. 14 Nevertheless death reigned from Adam until Moses, even over those who had not sinned in the likeness of the violation committed by Adam, who is a type of Him who was to come.
(Romans 5:12-14 NASB)
When Adam and Eve sinned, the human makeup was altered. Now, humans could know what evil felt like. And, in so doing, humans would face millions of momentary choices to do good or do evil. Combining this greater awareness with the natural weakness of human flesh, no mere human could, without fail, constantly resist the urge to choose evil.
And because of this, all sin, and all die.