Why were so many of the 500 witnesses still alive when Paul wrote 1 Corinthians?
Because only about 31 years had passed since the resurrection appearance. Paul wrote 1 Corinthians around A.D. 59, and many of the over 500 Jewish believers who saw the risen Christ at one time were still alive, perhaps only in their sixties, able to testify they had seen Him.
Les explains that after Christ appeared to Cephas, then to the twelve, He was seen by over 500 brethren at one time, Jewish believers in the Jerusalem area. Paul says the greater part remained to the present. Paul wrote this letter to the Corinthians about A.D. 59, roughly 31 years later. If some of those witnesses were in their late twenties or early thirties at the time, they would only be in their sixties yet, so they were still alive and could still tell others that they saw the resurrected Christ.