St. Thomas explains the idea of essential and accidental rewards:
“Merit as regards degree may be gauged in two ways.
“First, in its root, which is grace and charity. Merit thus measured corresponds in degree to the essential reward, which consists in the enjoyment of God; for the greater the charity whence our actions proceed, the more perfectly shall we enjoy God.
“Secondly, the degree of merit is measured by the degree of the action itself. This degree is of two kinds, absolute and proportional. The widow who put two mites into the treasury performed a deed of absolutely less degree than the others who put great sums therein. But in proportionate degree the widow gave more, as Our Lord said; because she gave more in proportion to her means. In each of these cases the degree of merit corresponds to the accidental reward, which consists in rejoicing for created good.”
Essential reward consists in the enjoyment of God himself; accidental reward consists in the enjoyment of created goods only. The difference between them is the merit arising from the grace and charity with which an act is performed, versus the merit of the act in itself.
This may not be the point that St. Thomas is making, but it seems at least a corollary that good acts performed without charity may earn one the reward of the enjoyment of created goods, but acts performed with charity merit the reward of the enjoyment of God himself. This latter is called an essential reward because God is charity (love) itself, whereas the goodness in created things is an accidental (not essential) goodness only.
Therefore the enjoyment of created goods can’t be the ultimate reward; the ultimate reward must be the enjoyment of what is good in its essence.