Why can alkoxides leave in the E1cb mechanism?
The fact that the elimination of $\ce{EtO-}$ leads to the generation of a carbonyl bond makes this a feasible reaction. Also, the $\ce{EtO-}$ ion generated can then take $\ce{H+}$ from water taking the equilibrium further in the direction of the forward reaction ( the net constant would be ~ $10^2$).