Because you cannot read the gospel accounts and not understand that they are presenting a literal resurrection. You just can't. It's not a matter of interpretation in this case.
There are many, many examples of where the NT books cannot be taken literally, leading to the conclusion that their substance lies not in the plots but in arguments about how to interpret the law and specifically what to do about the Roman crisis. These are the substance that matter, and the plot is a medium.
There are four canonized gospels for most Christians today, all ancient. The first in every bible sold is
Matthew which begins with a genealogy adjusted to make Jesus of the 42nd generation, purposely making the genealogy of
Matthew mismatch that in
Luke chapter 3. It is precisely a non-literal genealogy
Matthew begins with. Readers are expected to pick up on this special change to 42 generations, indicating the nature of the work is non literal. Lewis Carol picks up on this, making reference to it in his works as does Douglas Adams. Matthew stresses the importance that Jesus comes through 3 x 14 generations, yet none of the other gospels care about this. It is not literally important how many generations there are but that Israel has suffered enough. What is important is the idea that the suffering of Israel is for a purpose and is not a judgment against the people. One argument put forward is that suffering may not indicate impurity or a mistake but that the suffering may be for the benefit of the world. Son of man --> son of man. Jesus is the Son of Man in the story, and Israel is the Son of man in the prophets. Therefore the concept is what matters more than plot. Everything indicates the gospels are not meant to be bread and butter, plain books to be read by masses of non Jewish people.
From there
Matthew just keeps on going completely off of the expected path of a literal author. He uses the term 'Fulfill' in an ironic manner exactly the opposite of 'Fulfill'; and he does this ten times not just once or twice. If
Matthew were literal this would just be nonsense; but what he's doing is drawing a connection between the Son of man in the prophets and his character, Jesus. That's
Matthew, and it is the most obviously non-literal followed by
John, then
Luke, then
Mark in order of most obvious non-literalness. I have only touched on the beginning of problems for claiming a literal NT set.
The gospels directly contradict the Torah (most will recognize as the Mosaic Law) when Jesus says to believe in himself because of miracles. This absolutely cannot be literal.
Deuteronomy forbids this, unless I am mistaken in reading
Deuteronomy 13. A prophet is judged on his words, not on his miracles.
Deuteronomy is already canonized well before CE 0, and Jesus quotes from it in the gospels. In it Moses commands Israelites to challenge every prophet,
especially ones that do miracles, not to trust miracle workers. Jesus quotes from
Deuteronomy in the gospels. When he says to accept him because of his miracles he must either be joking or not literally speaking, because its not allowed for Jews to do that.
Judas Iscariot is a major sign of non-literalness. The death of Judas Iscariot (the betrayer of Jesus) is described as a suicide in
Matthew 27:5, but in
Acts 1:18 its described as a spontaneous death. He falls over, and his guts spill out. They are the same death, yet they are described very differently. Matthew's description is very significant, because in his version Judas is hung on a tree of his own volition, publicly cursing himself. In
Acts this does not happen, yet Judas death still indicates he was wrong to betray. That point was it was wrong to betray, not the specific manner in which Judas died. There are two endings but that same concept, and what was it that Judas represented? He was Iscariot (or a sicarri). He was a member of the Jewish assassins who were willing to kill Romans, opposing Roman government. The authors are suggesting not very subtly that this is a betrayal of Israel and that Israel should remain at peace with Rome and submit.