Because no talking snakes and magic fruits have ever been observed by science as far as I'm aware.
Science could not have observed it. Science is a matter of discovery by making use of sense knowledge and experimentation. But all that anyone knew of came from God teaching Adam and Eve. For instance.. Adam used leaves to cover his nakedness.. he didn't even know how to kill an animal to make coverings out of the fur.
In the same way that it's said that the antichrist will be completely possessed by the devil, to speak the devil's words.. then with that in mind it could be possible for the devil to walk and talk through the snake. With the understanding that it was later that the snake became a creature that slithered on it's belly and had a split tongue.
We can only imagine how different that snake animal was before it was transformed. For all that we know it could've looked like an upright creature with arms and legs. Perhaps it had a physical capability to talk to begin with. But it perhaps didn't have the level of cunning to achieve it on it's own.
So in order to get into the garden in disguise the devil lied to it as well. Probably told the creature that if it let the devil use it, that it would be wiser than it had been created to be. That is after all what the devil thought that he had achieved by disobeying God and his own status, he made himself free from God's rules. And so he set about to make himself better than he'd been created to be. He set out to get the authority away that God had given to Adam and Eve.. and thereby rule over all that's in the created heavens and earth. That authority reached up to but did not include the throne of God nor the actual habitation of God's heaven.
Satan visited there again when talking to God about Job.
As for the fruit on the tree of the knowledge of good and evil. The fruit wasn't magic. It was just fruit on the tree. Like fruit on any tree that we know of. Eve looked at the fruit. It looked good to eat. It was the devil that told her that she'd be wise if she ate it. The devil lied because he wanted them to disobey God when He told them not to eat from that tree.
Not eating from that tree is similar to tything in setting aside money to give to God instead of using it to go and buy some groceries.
It was the disobedience that caused the problem. There wasn't anything bad in the fruit. Eating it had not caused them to be subject to eventual death. The apple had not changed their nature from pure to sinful.
So, the tree was called the knowledge of good and evil. By their choices there was revealed the evil of the devil and the evil of their disobedience. If they'd obeyed God then that choice would've revealed their good which would have confirmed God's goodness working through them.
