If anybody ever learned רש”י and compared it to the מדרשים on many of the פסוקים, you see how careful רש”י was in what to cite and what not to cite. Often, there are three or four or five interpretations and he only quotes one! So, clearly, רש”י does not simply download – it’s not a מדרש-dump into רש”י. So, clearly, רש”י left a lot out.

Rabbi Nathaniel Helfgot, “What is פשוטו של מקרא?” YCT Yom Iyun (New York City: 13 January 2014).