Because that is what Paul says.
"What will their acceptance be but life from the dead."
They will accept Christ after this life.
They were really set aside for a particular purpose.
Christ is the firstfruits of the dead, so Paul is really saying that everyone will be brought back to life...
Not all Jews are saved,for some were disobedient,and whatever state a person dies in there is no reversal after that.If they die sins forgiven,they shall have eternal life,and if they have sins on their record,they will be separated from God for eternity.
People cannot be saved in the afterlife if they have sins on their record.Even people that claim Christ but had sins in their life that they did not get rid of will not make it,which Jesus said not all who say Lord,Lord,will be able to dwell with Jesus,but they will say why not,and Jesus will say because you were workers of iniquity,which they thought it was alright to have sins in their life,but be right with God.
Jud 1:5 I will therefore put you in remembrance, though ye once knew this, how that the Lord, having saved the people out of the land of Egypt, afterward destroyed them that believed not.
Jud 1:12 These are spots in your feasts of charity, when they feast with you, feeding themselves without fear: clouds they are without water, carried about of winds; trees whose fruit withereth, without fruit, twice dead, plucked up by the roots;
Jud 1:13 Raging waves of the sea, foaming out their own shame; wandering stars, to whom is reserved the blackness of darkness for ever.
The Bible says those that die with sins on their record are twice dead,physically and spiritually,there is no reversal in this case even if they are a Jew.
Jesus told the Jews who were not right,and did not accept Him,how can they escape the damnation of hell.