Why do scientists say that the creation story is made up?
Because it is.
The creation story, as stated in the bible, does not concur with ANY of the observations in a number of scientific fields such as Biology, Geology and Paleontology, to name the major ones. It doesn't even agree with recorded human history. Therefore any honest scientist can't come up with any other answer for you than "it IS made up".
Because many scientists believe in material realism that only natural cause exist and the Bible talks of supernatural causes.
The a priori assumption of many scientists is that anything that speaks of the supernatural must be false. This belief was expressed by Carl Sagan " The Cosmos is all is was or ever will be."
The belief that matter and energy are all that can exist and that everything happens by natural cause is not supported by empirical evidence. The empirical evidence is that matter and energy are not eternal is seen in the supernova experiments of 1997 and 1998,
String theory says that matter is actually vibrations of mathematical standing waves. This is consistent with the statements in the Bible that God spoke the universe into existence.
Speaking is vibrations.