Foods Containing Purines Why Are Exogenous Purines And Pyramidines Not Incorporated Into The Cell's DNA/ RNA?

Why are exogenous purines and pyramidines not incorporated into the cell's DNA/ RNA? - foods containing purines

Food purines and pyramidine. But the books and teachers, we have learned that the cell de novo synthesis or salvage path to acceptance.
But my question is:
Why exogenous purines and pyramidines are not included in the cell DNA / RNA?

My graduate who asked that question. I went through some books for Biochemistry and Molecular Biology. And even with Google. I need an answer to meet the students.