Why is there a resurrection? Why not leave them in heaven since Christianity teaches that paradise is in heaven?

There are plenty of Christian denominations that understand/teach about the earthly resurrection and new earth, it is in Luther's Small catechism, for example, and when I asked a baptist about it she explained that they belief that people return to the earth after the rapture.

• I am sure there are Christians that don't understand this.... There are churches that allow membership without any classes, and people generally have to ask if they want to know more than the basics... in addition, when people say heaven they generally just mean with the Lord, so even when the new heavens and the new earth are here we will still be in heaven... AND earth.
   
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  The following is quoted from a Baptist site:
  "The scriptures clearly show there is ONE BODY, ONE HOPE, and ONE FAITH for ALL true born-again Believers in Christ.Ephesians 4:4-6 There is one body (not two bodies, one earthly and one heavenly) , and one Spirit, even as ye are called in one hope (not two hopes, one earthly and one heavenly) of your calling; In the Eternal State (Rev 21) there will be an earthly, spiritual kingdom where God will be all in all.
  
  
  Part (2) If you were to die today, where would you spend eternity?
  Most born-again Christians would give the same answer---heaven.
  If saints are returning during the millennium to reign with Christ, how could that be correct?
  1Thess 3:13 To the end he may stablish your hearts unblameable in holiness before God, even our Father, at the coming of our Lord Jesus Christ with all his saints.
  
  Part (3) Where will those whose names are written in the Lamb's book of life spend eternity?
  First, they will be in heaven where Jesus is now.
  Matt 5:3 Blessed are the poor in spirit: for theirs is the kingdom of heaven.
  Second, they will return with Him at Armageddon.
  Rev 19:14 And the armies which were in heaven followed him upon white horses, clothed in fine linen, white and clean.
  Third, they will dwell with Him forever in His new earth.
  Matt 5:5 Blessed are the meek: for they shall inherit the earth.
  Rev 21:1-2 And I saw a new heaven and a new earth: for the first heaven and the first earth were passed away; and there was no more sea. 2 And I John saw the holy city, new Jerusalem, coming down from God out of heaven, prepared as a bride adorned for her husband."
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  This is from a Lutheran site: "You ask what the new heavens and earth will be like. I am not sure how to answer aside from emphasizing that it will be the eternal home of believers and thus a parallel term with "heaven," where God and his people will enjoy eternity together. To try to speak in great detail is inevitably going to be somewhat speculative and unnecessary. As Martin Luther once said, for us to engage each other with detailed descriptions of our eternal home would be like twin fetuses telling each other about how it will be in the world after their birth. They haven't experienced it yet and are wise to limit their speculations.
  The late Professor Siegbert Becker wrote these fitting words: "The most detailed description of this new heaven and new earth we find in the last two chapters of the Bible, where we have also a detailed description of the place where God's people will live through all eternity. John first tells us that he had a vision of a new heaven and a new earth. . . . What more can we say to add anything to that picture of endless bliss and glory? We can only join in the prayer of St. Bernard, 'Jesus, in mercy bring us to that dear land of rest.' "