Because God loves us and does not want us to remain enslaved to sin and death that we all have chased after. So great was that love that Jesus was willing to face death himself - facing, and defeating the whole of what has defeated us, so that he has become our salvation, our means of reconciliation to God.
I do believe in and hope for the restoration of all things under [growth of all things into and under] the Lordship of Christ. I don’t know the details of what that looks like, and don’t buy into the popularized versions of such events that have captured the imaginations of (probably mostly American) audiences over the last century or two.
God makes all things new, including you and me. The old needs to pass away to make room for the new. That doesn’t mean there is no continuity with the old. In other words (just as with our physical bodies - temples that they are) there is a clothing of our mortal frames with the immortal, as Paul writes to the Corinthian church. I believe that is a model for all creation that groans and awaits our salvation along with us. There is an “already/not yet” aspect to all this, because we are told that the kingdom of God is already here among us (Luke 17). That doesn’t means it’s fully realized or has reached its final culmination.
As to details and timelines of how that all works out - I remain ignorant and trusting in God that it is and will continue to unfold in God’s plan, without my needing to pretend full understanding or needing to chase after various traditions and theologies that demand particular understandings and traditions of men as a precondition for joining their club. All we need to do is look to Christ and learn faithfulness and obedience to Him. While doing that, all else will be worked out for us as needed, through our death and beyond. Lord, help us to live by that trust and hope - thereby learning to Love and Live truly.