Because people aren’t born as a blank slate, although people seem to like that idea. Genetics play a huge role in personality and character. Some people are born without remorse. They need help and therapy like every other type of disability. But people are just too hung up on the idea of free will and virtuous character values to accept that our brains are organs that can have broken parts.
When you happen to cross paths with someone like that you will know. A kid I know is like this. He would hurt his younger brother to get attention and use other manipulation tactics (at 8 years old!). He will lie straight to your face and it’s just obvious he is very different from other children.
His mother had to stop working and basically 24/7 supervise this kid and the overall situation is nightmarish.