כִּֽי־עִמְּךָ֥ הַסְּלִיחָ֑ה לְ֝מַ֗עַן תִּוָּרֵֽא׃
그러나 사유하심이 주께 있음은 주를 경외케 하심이니이다
Rashi on Psalms
For forgiveness is with You You did not give permission to an agent to forgive, as it is said (Exod. 23:21): “for he will not forgive your transgression.”
Rashi on Psalms
in order that You be feared for this, so that no man will rely on the forgiveness of another. (I found this.)