This is because the idea of ש ַּבָּת-Shabbos is that “on it Hashem ש ָבַת-rested.” (Bereishis 2:3) The word ש ב ָּת-Shabbos comes from the word ש ָּב ת-rested.
By way of analogy: When a person rests from his work, his intellect (mental focus) returns to its source to contemplate their original motivations. While he was working, his mind was busy with his work. Afterwards, when he rests from his work, this mental focus reverts to its source deeper in the soul.
So too regarding divine revelation, it is written (Shemos 20:9) “It’s a day of Shabbos/Rest for Havaya (Hashem).” Meaning: The aspect of divine revelation called “Havaya” which “ריִעֵּ שִמ ךְתאֵצְּב -comes out to us through a hairsbreadth revelation” to make creation exist from nothing, on Shabbos returns to its source in divine motivation that precedes creation, as it says, “then you will have pleasure on Havaya (from Hashem),” meaning from a level of divine pleasure that is above and precedes the level of Havaya.
Therefore, on Shabbos we do not say the request during Shemona Esray “forgive us Hashem”, since there is no involvement in spiritual refinement and removal of unholiness. The focus is on the pleasure in connecting to Hashem.
