Philip S. Alexander, "Midrash," A Dictionary of Biblical Interpretation, ed. R.J. Coggins and J.L. Houldin (SCM Press, Trinity Press, 1990) 454, 458.
The reason the midrashim have this anonymous character is basically ideological. The darshanim saw themselves as engaged in a collective enterprise, as working within a tradition. Their task was to pass on what they received, and to contribute to the development of the oral Torah. This left little scope for individuality or idiosyncrasy. The darshanim did not set great store by originality, and if innovation were required it had to be done in terms of pre-existent tradition. Nor did they have a strong proprietorial attitude towards their ideas: whatever they contributed to the sum of the tradition became at once public property and could be used freely and adapted by others. (454)
The darshan does not stand before the text of scripture with absolute freedom. He must work within that tradition. He is forbidden to disclose in scripture aspects which are not in accordance with the halakhah. He can only interpret scripture aright if he has studied in the right schools, with the right masters who stand in an unbroken line of tradition going back to Moses himself. (458)
This is different from modern academia, and yet similar. Modern academia encourages originality, yet it wants students to interact with the thoughts that have come before.
I've heard the thoughts in these quotes before, but not in a long time. Some professors have maintained that ancient interpreters tried to create something new while appearing continuous with the old. After all, if they thought that the old was sufficient, then why'd they see the need to write the new? Some may say that they were merely trying to clarify the ambiguous old. The question then becomes, "When do they do that, and to what extent?" I think James Kugel does well to point out examples in which ancient interpreters aimed to clarify the ambiguities of Scripture, although there are times when the ambiguities he claims to identify appear rather artificial. At the same time, was I Enoch designed to clarify an ambiguity? It basically takes a character from Genesis 5 and runs with him, using him as a launch-pad for its own agenda! I Enoch's connection with the "old" looks pretty thin indeed!