In order to receive the reward we must be fit vessels to receive it. The reward is closeness to Hashem. In the spiritual sense, closeness is similarity. Hashem is a pure Giver. We must become givers. What is the most precious thing we can give? Ourselves, our free will, giving it to Hashem and giving to His children. Only then can we become, in a manner of speaking, like Hashem.
To be created to just receive, to purely take, is the opposite of Hashem and would be an inherent, intrinsic contradiction to closeness with Hashem.
Now if you answer:”But Hashem can do anything, why did He create the Universe like this?” then no answer will answer any of your questions.