Because it is reasonable to suggest a headmaster adopt students that have troubled pasts or hard situations. There would be many. Surely Dumbledore isn’t responsible for every child’s well-being.

It’s also not a great take to say Tom would’ve turned out differently. By the time Dumbledore met him, Riddle was mostly aware of his abilities and seemed happy using them to hurt people when he wanted. Dumbledore sensed this. Hogwarts was the chance for Tom to turn it around anyway and he didn’t. He instead used the time and resources to further his journey into becoming Voldemort.

It’s not on the village to redeem a person. They have to chose it.