Uhm... Because the history of the Old Covenant (Jewish priests offering sacrifice) and the history of the New Covenant (priests offering sacrifice) and the theology of both books, demand it.
4,000 plus years. Truth cannot contradict itself. The Church is visible--not invisible. There is one Lord, one Faith, one baptism. The Faith must have authentic living interpresters.
"Uhm... Because the history of the Old Covenant (Jewish priests offering sacrifice) and the history of the New Covenant (priests offering sacrifice) and the theology of both books, demand it."
Ah, but therein lies the rub. The New Covenant in His blood obviates the need for further priestly sacrifices. He, as our High Priest forever, offered Himself once, for all, for all time, for all sins as the only acceptable sacrifice which could wash away sins. It is written that the blood of bulls and goats cannot wash away sin, but the blood of the Messiah can. Can any priest, bishop, archbishop, pope, or high priest offer a more worthy sacrifice? Then why should they even try? Hebrews 10 records: "For the Law, since it has only a shadow of the good things to come and not the very form of things, can never, by the same sacrifices which they offer continually year by year, make perfect those who draw near. Otherwise, would they not have ceased to be offered, because the worshipers, having once been cleansed, would no longer have had consciousness of sins? But in those sacrifices there is a reminder of sins year by year. For it is impossible for the blood of bulls and goats to take away sins." The OT sacrificial system did not take away sins. It only covered them. Jesus was made sin for us that we might be made righteous in the sight of God. So, why do we need intercessory priests to speak to God for us? We are authorized to approach Him with joy and thanksgiving covered by the blood of Jesus.