Why is circumcision no longer required for salvation if Gentiles are “grafted into” Israel (Romans 11)?

In Romans 11, Paul describes Gentile believers as being “grafted in” to the olive tree of Israel. If Gentiles are joined to Israel spiritually, why does circumcision — a covenant sign originally given to Abraham and his descendants (Genesis 17) — no longer apply as a requirement for salvation or covenant membership? How do Christian traditions theologically reconcile the Abrahamic circumcision command with the New Testament statements that circumcision is not required (Acts 15, Galatians 5)?