That's easy. There is not one single place in the Word Of God where it tells us that "when we die we will go straight to heaven". If every Saint whom has ever died has gone straight to heaven, Who will Jesus raise from the grave when he comes back to raise us from the grave??? If we are already in heaven then our graves would be empty when Jesus returns.
1Co 15:50 Now this I say, brethren, that flesh and blood cannot inherit the kingdom of God; neither doth corruption inherit incorruption.
1Co 15:51 Behold, I shew you a mystery; We shall not all sleep, but we shall all be changed,
1Co 15:52 In a moment, in the twinkling of an eye, at the last trump: for the trumpet shall sound, and the dead shall be raised incorruptible, and we shall be changed.
1Co 15:53 For this corruptible must put on incorruption, and this mortal must put on immortality.
1Co 15:54 So when this corruptible shall have put on incorruption, and this mortal shall have put on immortality, then shall be brought to pass the saying that is written, Death is swallowed up in victory.
So according to the scriptures above the Word Of God tells us that we can not go to heaven unless we die and shed our outer shell of flesh and blood and become incorruptible.
Secondly these scriptures tell us that we must put off mortality and put on immortality BEFORE WE CAN BE ACCEPTED INTO HEAVEN. The question is, If we are already in heaven when Christ comes back to raise us from the grave so we can become transformed from corrupted into incorrupted and from mortal to immortal, will we need to come back to the grave from heaven, remove incorruptible and immortality, put back on corruption and mortality so Christ can raise us from the grave so we can change from corrupted and mortal and put back on immortality and incorruption????? I don't think so.
I submit to you that we do not go to heaven as soon as we die but that we stay in the grave as mortal and corrupted until Christ comes back to raise us from the grave on Resurrection Day in incorruption and immortal.