This is because people are either servants of God or servants of other human beings. Both servant and slave have the same two characteristics: work and obedience.
A servant or slave is one who follows orders and does everything his master tells him or her to do. Slavery should still exist because we should obey every commandment of Jesus Christ.
A master has full claim over his slave, and the slave has full commitment to his master.
Throughout his epistles, Paul calls himself a slave because it was his title of honor. Paul could have called himself many things, but at the beginning of most of his books, he identified himself as a slave, a servant or a bondservant of Jesus Christ (Romans 1:1; I Corinthians 7:22; Philippians 1:1; Titus 1:1).
Paul, a slave of Jesus Christ, called to be an apostle, set apart for the gospel of God... (Romans 1:1)
Slavery should still exist! However, we should not be a slave of man Instead, we should be a slave and a bondservant of Jesus Christ just as Paul was.