If freedom and full human flourishing/fulfillment without sin/pain/death was inherent in the Garden of Eden and will be again in the New Heavens and New Earth, then why is sin/pain/death necessary in this life?

The crux of this inquiry comes from the common assertion I hear from Christians that full human flourishing/fulfillment can only exist in an environment where evil, death and great loss/pain are possible. But, evil will not be possible in the New Heavens and New Earth. And many people will be there that did not have the opportunity to choose to be there (for example, miscarried babies, infants, young children and mentally ill or mentally challenged people), so the claim that the travails of this life are worth it (because of potential character formation) doesn’t apply to them.