Because the writers and re-writers of the Bible were and are human.
The following is John 3: 10-21, which includes the famous saying from John 3: 16. This is how the New King James Version of the Bible re-wrote the scripture:
10 Jesus answered and said to him, “Are you the teacher of Israel, and do not know these things? 11 Most assuredly, I say to you, We speak what We know and testify what We have seen, and you do not receive Our witness. 12 If I have told you earthly things and you do not believe, how will you believe if I tell you heavenly things? 13 No one has ascended to heaven but He who came down from heaven, that is, the Son of Man who is in heaven.[a] 14 And as Moses lifted up the serpent in the wilderness, even so must the Son of Man be lifted up, 15 that whoever believes in Him should not perish but[b] have eternal life. 16 For God so loved the world that He gave His only begotten Son, that whoever believes in Him should not perish but have everlasting life. 17 For God did not send His Son into the world to condemn the world, but that the world through Him might be saved.
18 “He who believes in Him is not condemned; but he who does not believe is condemned already, because he has not believed in the name of the only begotten Son of God. 19 And this is the condemnation, that the light has come into the world, and men loved darkness rather than light, because their deeds were evil. 20 For everyone practicing evil hates the light and does not come to the light, lest his deeds should be exposed. 21 But he who does the truth comes to the light, that his deeds may be clearly seen, that they have been done in God.”
The writers put the quotation marks in such a way that Jesus himself is saying, “For God so loved the world that He gave His only begotten Son, that whoever believes in Him should not perish but have everlasting life.” But after confusedly reading it a few times, it’s obvious to me that Jesus stops speaking at the end of verse 12. Why? Because in verse 13, the writer (and it’s not Jesus), says about Jesus that he has already ascended to Heaven. Well that does not happen until after Jesus has been crucified, lies in a cave for three days, rises from the darkness and appears to his disciples.
Of course, if you wanted every reader to believe that “belief in Jesus” in and of itself sends you straight to heaven, you would attribute John 3: 16 to Jesus himself. This would be absolutely necessary to convince careful readers of the Bible, since any reader can see that Jesus contradicts this “just believe” concept many times in Jesus’s own words. And when I say his own words, I mean words that are both attributed to him and make sense that they are attributed to him, unlike John 3: 16.
It is obvious to me, the heathen, that Jesus’s actual speech ends at the end of verse 12, and that the writer (let’s assume it is one person) then begins to tell the reader of his own views about Jesus and God. There should be a paragraph break here, between 12 and 13 in order to indicate this change in speaker. But there is not.
The compilers of the New King James Version decided to attribute the rest of the chapter, verses 13 to 21, to Jesus himself, as his quotable words, even though he did not say them. So, the compilers have Jesus telling people that all they have to do is believe, and that all believers will reach Heaven where the only one who has ascended there resides. And the compilers also have the readers believe that Jesus himself said that those who do not believe are already condemned. This means that they can do nothing to change their fate; this would be a good belief for those going to war with nonbelievers -- they’re going to hell anyways, so why not kill them now?
The point of all this is not necessarily to say that the Protestants are wrong and that the Catholics are right, even though it does seem that way according to the words of Jesus. My point here is that people are constantly twisting the words of the Bible for their own use. But you knew that already.
To be more specific, people need to feel that they are unconditionally loved and accepted and safe. And they need to feel free from guilt and responsibility. These are basic human instincts. Why? To put it succinctly: because it feels good, and it frees us.
How better to free us from responsibility and guilt but by telling us that all we have to do is believe? How better to make us feel okay about keeping almost all our wealth to ourselves? After all, Jesus just wants you to believe! Do what you will with your life, with your money, with your time -- you are free now that you have accepted Jesus as your personal savior.
And how better to increase the the numbers of believers but to tell people that nonbelievers go to hell, according to Jesus himself, and that all you have to do to get to heaven is believe, according to Jesus himself?
In this single example -- the writing and re-writing of John 3: 16 -- you have institutions who want larger, wealthier churches, bigger congregations; you have people who dearly wish to believe that they are free to do what they wish as long as they believe. The individual and the institutional instincts come alive and interpret and re-write the Bible accordingly. People are free; they do exactly what Jesus demands them not to do. They do exactly what Jesus says will send them to hell -- they turn their backs on those in need. Why? Because according to some other part of the Bible, re-written or not, Jesus does not really care what you do; just believe. It’s hokum, but it’s the hokum that makes us feel good about ourselves.
And that is why there is still hunger in the world.