Another interpretation: Moses said to Him, "Master of the universe, I know that You love Your children, and that You seek only someone who will plead an advocacy (sanegoria) on their behalf," as it is said, "And now, let Me be." And was He being held, that He should say "let Me be"? Rather, He sought only someone who would seek mercy for them.
God Wanted a Defender for Israel, Not a Bystander
Yalkut Shimoni on Torah 392:3
דָּבָר אַחֵר. אָמַר לוֹ מֹשֶׁה. רִבּוֹנוֹ שֶׁל עוֹלָם, יוֹדֵעַ אֲנִי שֶׁאַתָּה אוֹהֵב בָּנֶיךָ וְאֵין אַתָּה מְבַקֵּשׁ אֶלָּא מִי שֶׁיְּלַמֵּד עֲלֵיהֶן סָנֵגוֹרְיָא, שֶׁנֶּאֱמַר וְעַתָּה הַנִּיחָה לִי וְכִי תָּפוּס הָיָה בּוֹ אֶלָּא אֵינוֹ מְבַקֵּשׁ אֶלָּא מִי שֶׁיְּבַקֵּשׁ עֲלֵיהֶן רַחֲמִים.