First of all, the returning of people to this world after death defeats the whole purpose of this universe’s creation and the appointment and sending of the prophets. Secondly, if by any chances people were to come back, there is no guarantee that they would be better, because the world they used to live in would be the same old world and they would be the same old people with the same temptations and desires they used to have. Just like before, they would know that these desires are all like a mirage, yet they would still fall in love with them and do any evil acts to reach these pleasures.
It is God’s will for everything to reach perfection and development in this world. He has created man in this world and supplied him with intellect and divine revelation in order for him to move towards perfection. Therefore, if people were to be allowed to return to this world after death according to their own will, the whole purpose of creating the universe and sending prophets would be defeated and such thing doesn’t comply with Allah’s norm and routine practice.
Moreover, there is no guarantee that such individuals will change for the better after returning, because they will be in the situation they were before; the same world, the same desires, pleasures, satanic desires, self-pride and rebellion. In other words, the same factors causing them to go astray and deny Allah’s signs today, will be there if they ever return, causing them to go astray once again.
That is the reason behind why the Quran says: (In Falsehood will they be) Until, when death comes to one of them, he says: "O my Lord! send me back (to the world),-
"In order that I may work righteousness in the things I neglected." - "It isn’t so! It is but a word he says (and if he is returned he will be as he was before)."- Before them is a Partition till the Day they are raised up.
What is important here and should be noted is that in response to their request, the word kalla (it isn’t so) is used. In Arabic literature, this word is used for two things:
1- preventing and not allowing something 2- annulling the opposite side’s argument and claim. There is a big chance that this verse is trying to get both meanings across at the same time with this single word. In this case, the verse’s meaning will go like this: First of all, their request is merely an excuse for returning, but such a thing is impossible. Secondly, their claim is a false one when they say that they will make up for the lost past and do righteous deeds. Allah says what they are saying isn’t true and that if they go back, they will be as they were before. In addition to the fact that the word la’alla (meaning maybe) in the verse shows that even these individuals themselves aren’t sure about their own future if they come back and doubt that they will change and probably know that their regret and repentance is because of the fact that they are edging ever closer to death.
Anyways, what is important is that returning is useless. Allah says in the holy Quran: “If they were returned (to this world) they would repeat the actions that they had been ordered not to commit”, therefore their awakening and being after the truth is only temporary and isn’t continuous.
These people are like the kafirs and sinners who when getting stuck in a sea storm and get very close to death, ask Allah (s.w.t.) for help, yet after being saved, they return to the same state they were before. In other words, these people have most probably comprehended the fact that this world isn’t forever, and have seen death with their own eyes and yet they still lose their will when it comes to worldly pleasures and desires, and are ready to do anything, no matter how bad, in order to satisfy these desires.
 Al-Mizan, v.7, pg. 84.
 “حتی اذا جاء احدهم الموت قال رب ارجعون. لعلی اعمل صالحاً فیما ترکت کلاّ انّها کلمة هو قائلها” Mo’minum:99-100
 See: Tafsir Nemooneh, v.14, pg. 313, Al-Mizan, v.5, pg.98.
 Tafsir al-Tebyan, v.7, pg.393, Kanz-ul-Irfan, v.9, pg.212.
 “ولو رودا لعادوا لمانهوا عنه” An’am:28
 Tafsir Nemooneh, v.5, pg.200.