Perhaps it's because you have an incomplete Bible.
So he armed every one of them, not with defence of shield and spear, but with very good speeches, and exhortations, and told them a dream worthy to be believed, whereby he rejoiced them all. Now the vision was in this manner. Onias, who had been high priest, a good and virtuous man, modest in his looks, gentle in his manners, and graceful in speech, and who from a child was exercised in virtues holding up his hands, prayed for all the people of the Jews. After this there appeared also another man, admirable for age, and glory, and environed with great beauty and majesty. Then Onias answering, said: This is a lover of his brethren, and of the people of Israel: this is he that prayeth much for the people, and for all the holy city, Jeremias, the prophet of God. Whereupon Jeremias stretched forth his right hand, and gave to Judas a sword of gold, saying: Take this holy sword, a gift from God, wherewith thou shalt overthrow the adversaries of my people Israel. Thus being exhorted with the words of Judas, which were very good, and proper to stir up the courage, and strengthen the hearts of the young men, they resolved to fight, and to set upon them manfully: that valour might decide the matter, because the holy city, and the temple were in danger. 2 Machabees 15: 11-16
How can Jeremiah talk to Judas if Jeremiah was dead?
Jeremiah (dead) talked to Judas (alive). This is vision ( 2 Machabees 15:12 Now the vision was in this manner...) not a prayer of a living person to someone dead.
You might note that Judas prayed only to God in this chapter:
2 Machabees 15:26-27, But Judas, and they that were with him, encountered them, calling upon God by prayers: 27 So fighting with their hands, but praying to the Lord with their hearts...
Revelation 7:
9 After this, I saw a great multitude, which no man could number, of all nations and tribes and peoples and tongues, standing before the throne and in sight of the Lamb, clothed with white robes, and palms in their hands.
10 And they cried with a loud voice, saying: Salvation to our God, who sitteth upon the throne and to the Lamb.
11 And all the angels stood round about the throne and the ancients and the four living creatures. And they fell down before the throne upon their faces and adored God,
12 Saying: Amen. Benediction and glory and wisdom and thanksgiving, honour and power and strength, to our God, for ever and ever. Amen.
13 ¶ And one of the ancients answered and said to me: These that are clothed in white robes, who are they? And whence came they?
14 And I said to him: My Lord, thou knowest. And he said to me: These are they who are come out of great tribulation and have washed their robes and have made them white in the blood of the Lamb.
15 Therefore, they are before the throne of God: and they serve him day and night in his temple. And he that sitteth on the throne shall dwell over them.