Because Hebrews 5:1 says that every high priest is taken out of man. Melchizedek was a man just as Jesus was a man. He was without father and mother in the same way Jesus was without father and mother. He was without beginning of days or end of life in the same way Jesus was without beginning of days or end of life. Yet, both men had an earthly genealogy and both men were born into this world and died. Whatever these terms define for the one they also define for the other. Since Jesus had a natural mother and a natural birth and a natural death then what changes must be our understanding of how scripture uses these terms in connection with the priesthood. Jesus must fulfill the type.
I think you need to move on to Hebrews 8 and confirm the "position" of HIGH PRIEST FOREVER was filled...FOREVER...(that there is no longer any priesthood or earthly high priest that we are waiting for)...that POSITION was filled and there are NO VACANCIES...because if CHRISThad been just another priest serving in an earthly temple doing the works of an earthly high priest, ministering for the sins of the people, HE would have to continually offer up sacrifices over and over...but CHRIST, by HIS SACRIFICE ONCE and FALL entered into the HOLY OF HOLIES through the TRUE VEIL...HIS BODY...and those who through faith in HIS BLOOD, are covered over by HIS BLOOD and can also enter into the Presence of GOD because of THAT COVERING...
Even Moses had to duplicate the earthly temple which was simply a shadow and picture of the TRUE HEAVENLY ONE...
I think you need to read Hebrews 5 through 8 in context to understand that this letter to the Hebrews is even in service to those Jews who will be (in the last days) wondering about the earthly sacrifices being reinstated in an earthly temple...and I pray that they may be reminded of Haggai 2 and understand that there are none clean before GOD...I pray that they will remember GOD'S WORD and understand that if all are declared unclean, than who can offer up a sacrifice in the sight of GOD (when that earthly temple is rebuilt)?
NONE...for if all are declared unclean, that means that any earthly presider who dares to offer up the sacrifice (even if that sacrifice is "perfect") he, by being declared unclean renders the sacrifice (because of his own unclean hands) unacceptable...
And I pray that they may be given the understanding that CHRIST JESUS as HIGH PRIEST FOREVER, entered into the HOLY OF HOLIES by HIS HOLY SACRIFICE, (the TRUE VEIL, which is HIS BODY) in the sight of GOD...and because HE is HOLY AND SET APART to GOD for THIS VERY prophecied WORK, (for even GOD, through Abraham, reenacted this "sacrifice of a promised son whom he loved), HE alone offered up the ONE AND ONLY HOLY AND ACCEPTABLE OFFERING (in the sight of GOD) and REMAINING FOREVER the HOLY PRESIDER over THAT OFFERING...