Because not everyone was lucky enough to be born sinless and maintained by virtue of double predestination.
BTW, Why do Calvinists claim for themselves what they deny to Mary, the Mother of God?
lol. Calvinists know all men are sinners while it is the RCC who believes Mary was born sinless.
Jesus Christ is the only person ever born sinless. As Paul reminds us, "while we were yet sinners, Christ died for us."
Read your Bible and maybe you won't keep repeating these mistakes.