Here is the main point: Our High Priest sat down in the place of highest honor in heaven, at God's right hand.
There he ministers in the sacred tent, the true place of worship that was built by the Lord and not by human hands.
And since every high priest is required to offer gifts and sacrifices, our High Priest must make an offering, too.
If he were here on earth, he would not even be a priest, since there already are priests who offer the gifts required by the law of Moses.
They serve in a place of worship that is only a copy, a shadow of the real one in heaven. For when Moses was getting ready to build the Tabernacle, God gave him this warning: "Be sure that you make everything according to the design I have shown you here on the mountain."
References for Hebrews 8:5
But our High Priest has been given a ministry that is far superior to the ministry of those who serve under the old laws, for he is the one who guarantees for us a better covenant with God, based on better promises.
If the first covenant had been faultless, there would have been no need for a second covenant to replace it.
But God himself found fault with the old one when he said: "The day will come, says the Lord, when I will make a new covenant with the people of Israel and Judah.
This covenant will not be like the one I made with their ancestors when I took them by the hand and led them out of the land of Egypt. They did not remain faithful to my covenant, so I turned my back on them, says the Lord.
But this is the new covenant I will make with the people of Israel on that day, says the Lord: I will put my laws in their minds so they will understand them, and I will write them on their hearts so they will obey them. I will be their God, and they will be my people.
And they will not need to teach their neighbors, nor will they need to teach their family, saying, 'You should know the Lord.' For everyone, from the least to the greatest, will already know me.
And I will forgive their wrongdoings, and I will never again remember their sins."
References for Hebrews 8:12
When God speaks of a new covenant, it means he has made the first one obsolete. It is now out of date and ready to be put aside.