Here's why:

2 Corinthians 5:6-8

"Therefore, being always of good courage, and knowing that while we are at home in the body we are absent from the Lord, for we walk by faith, not by sight, but we are of good courage and prefer rather to be absent from the body and to be at home with the Lord."

Paul expected to be with the Lord upon death, not waiting in a grave for thousands of years.

Philippians 1:23

"But I am hard-pressed from both directions, having the desire to depart and be with Christ, for that is very much better."

Paul did not speak of soul sleep or a long delay, but an immediate presence with Christ.

2 Timothy 4:6-8

"The time of my departure has come... there is reserved for me the crown of righteousness..."

Paul knew his death was near and anticipated receiving his reward shortly after.

Revelation 20:4-6 (speaking of the first resurrection) already took place before the final judgment in AD 70, as part of the covenant transition.

Paul, along with the faithful departed, received what was promised when the resurrection and judgment took place at the end of the Old Covenant age (Hebrews 11:39-40, Matthew 24:3, 34).

Conclusion:

Apostle Paul is not in the grave. He is already with Christ, just as he hoped and wrote. The resurrection has been fulfilled, and those in Christ who died before AD 70 were raised as promised.