People did not have dna tests in those days and you needed 4 witnesses. That is why they had this ruling.
The only way for you to reject the child is through Li'aan in which you swear that if you are lying that the curse of Allah befalls you or the person lying. Therefore, you needed to be 100% certain.
After speaking to sheikh this is true. The man has to swear iys not his to relief the responsibility from himself otherwise kid is his