> It also suggests that focusing on a rewarding task takes a great deal of energy, and that it may be something that can be improved, especially in individuals with attention deficits

So, to get the reward you have to spend energy first, and this energy expenditure is greater than you spend on other tasks (that will not provide rewards). And if you tweak that equation (a very small and instantaneous reward that doesn't require energy - like infinite scroll in some social media), focus goes out the window.