Because it is true, because the Holy Spirit - promised to the Church to guide it in all truth to fulfill the command "Go forth and teach all nations" - led the Church to incorporate into the canon.

If Jesus is God, and the Word Incarnate in a Triune God, and Man is fallen and awaiting redemption, it makes complete sense for Him to breach the veil of the heavenly Temple and offer Himself as our High Priest to the Father.

He described Himself as a priest by referencing all those OT scriptures which you suggested no one was familiar with.

No, He did not provide a four-page theological manual in English for us to cram for the test.