The reason possession is illegal is to try to prevent further production of it and to eliminate continuing abuse to children. The theory is that allowing even mere possession perpetuates a market of “buyers” that would further stimulate production. This is well covered in the Congressional findings associated with 18 U.S.C. 2251. See e.g. https://www.law.cornell.edu/uscode/text/18/2251 (click the notes tab).