Jesus Said ”IT IS FINISHED” So Why Do Catholic Priests REPEAT the Sacrifice of Christ?

The sacrifice of Jesus Christ on the cross was the greatest event in human history! That is when Christ took the SINS of humanity upon HIMSELF. The last words that he spoke before He died were "IT IS FINISHED" which means that he accomplished exactly what He came to do. Three days after that death on the cross Jesus rose from the dead just like He said he would. That death on the cross was prophesied in the Old Testament and it happened ONCE and it can never be REPEATED again. NEVER! So why do the Roman Catholic priests continue to offer up the "sacrifice of the Mass" which they claim to be the SAME sacrifice as that of Christ on the cross?