Jamieson, Fausset and Brown on Hebrews 7:8

8. Second point of superiority: Melchisedec's is an enduring, the Levitical a transitory, priesthood. As the law was a parenthesis between Abraham's dispensation of promise of grace, and its enduring fulfilment at Christ's coming (Ro 5:20, Greek, "The law entered as something adscititious and by the way"): so the Levitical priesthood was parenthetical and temporary, between Melchisedec's typically enduring priesthood, and its antitypical realization in our ever continuing High Priest, Christ.

here--in the Levitical priesthood.

there--in the priesthood after the order of Melchisedec. In order to bring out the typical parallel more strongly, Paul substitutes, "He of whom it is witnessed that he liveth," for the more untypical, "He who is made like to Him that liveth." Melchisedec "liveth" merely in his official capacity, his priesthood being continued in Christ. Christ, on the other hand, is, in His own person, "ever living after the power of an endless life" (He 7:16, 25). Melchisedec's death not being recorded, is expressed by the positive term "liveth," for the sake of bringing into prominence the antitype, Christ, of whom alone it is strictly and perfectly true, "that He liveth."