# Why does a carbonyl reform in ester reactions

I have noticed that frequently whenever esters react with a nucleophile, the nucleophile will attack the carbonyl carbon and eventually in order for the carbonyl to reform the other oxygen will leave for example the first addition of the grignard reagent to an ester: $\ce{PhMgBr + Ph-(C=O)-O-R}$ will result in $\ce{(Ph)2-(C-O^{-})-O-R}$ as an intermediate, and then rather than the $\ce{O^{-}}$ simply getting protonated with water for example, its favorable for the $\ce{O-R}$ to leave and the $\ce{C-O^{-}}$ to become $\ce{C=O}$. Why is the carbonyl reforming more favorable, isn't $\ce{O-R}$ a bad leaving group?