For several reasons.
First of all Jesus said that Noah was a real person:
“ But as the days of Noah were, so shall also the coming of the Son of man be.”
Jesus' second coming will be a real event, not some parable.
Secondly , Noah was included in a genealogy by the evangelist Luke.
You don’t put fictitious characters into genealogies.
And the most important thing, if one is going to turn the book of Genesis into an analogy or parable, then in effect what one would be saying is that there really was no Adam and Eve, no Garden of Eden, no fall of man, no introduction of sin, and thus no need for Christ to come and save anybody.
And the Apostle Paul would be a liar, for he taught that sin came into the world because of Adam.
The entire Old and New Testaments would be nothing but a compilation of lies and myths.
A position quite suitable for an Atheist, Agnostic or non Judeo-Christian person
But not for a Jew or Christian.