Because the story in the Bible is clearly talking about a
global flood.
Genesis 6:9-9:17
9 This is the account of Noah and his family.
Noah was a righteous man, blameless among the people of his time, and he walked faithfully with God. 10 Noah had three sons: Shem, Ham and Japheth.
11 Now the earth was corrupt in God’s sight and was full of violence. 12 God saw how corrupt the earth had become, for
all the people on earth had corrupted their ways. 13 So God said to Noah,
“I am going to put an end to all people, for the earth is filled with violence because of them. I am surely going to destroy both them and the earth. 14 So make yourself an ark of cypress[
a] wood; make rooms in it and coat it with pitch inside and out. 15 This is how you are to build it: The ark is to be three hundred cubits long, fifty cubits wide and thirty cubits high.[
b] 16 Make a roof for it, leaving below the roof an opening one cubit[
c] high all around.[
d] Put a door in the side of the ark and make lower, middle and upper decks. 17 I am going to bring floodwaters on the earth to destroy all life under the heavens, every creature that has the breath of life in it.
Everything on earth will perish. 18 But I will establish my covenant with you, and you will enter the ark—you and your sons and your wife and your sons’ wives with you. 19 You are to bring into the ark two of all living creatures, male and female, to keep them alive with you. 20 Two of every kind of bird, of every kind of animal and of every kind of creature that moves along the ground will come to you to be kept alive. 21 You are to take every kind of food that is to be eaten and store it away as food for you and for them.”
22 Noah did everything just as God commanded him.
7 The Lord then said to Noah, “Go into the ark, you and your whole family, because I have found you righteous in this generation. 2 Take with you seven pairs of every kind of clean animal, a male and its mate, and one pair of every kind of unclean animal, a male and its mate, 3 and also seven pairs of every kind of bird, male and female, to keep their various kinds alive throughout the earth. 4 Seven days from now I will send rain on the earth for forty days and forty nights, and
I will wipe from the face of the earth every living creature I have made.”
5 And Noah did all that the Lord commanded him.
That's a description of a global flood, not a local flood.