Yalkut Shimoni, Tehilim 247, has a question. Why does it say about Ya'akov that he wlll rejoice, and about Yisrael that he will be glad? Why are the other Avot, Avraham and Yitschak, not mentioned in Tehilim 14? The teruts given by Yalkut Shimoni is not important.
What is important is the question. The deeper question is why Avraham and Yitschak are not mentioned anywhere, doing anything, in the first book of Tehilim? And the answer seems obvious: David did not know them.
This would also answer the question of Yalkut Shimoni, but, well, it is not a very orthodox answer.
