It could be related to risk and energy expenditure. Moving faster requires more energy, and the farther away your piece of cheese is, the more likely it is that something unexpected might happen to take it away/stop you before you get to it.
Add to this that there can always be more pieces of cheese to find, and always running from one to the next one could not be your best course of action.
It's also that short distances are more accurately judged than long ones (ditto endurance over short distances). There are often sprints to the finish in running races when there's a final surge of adrenaline, when you realise you can sustain an increased pace all the way to the end.
Add to this that there can always be more pieces of cheese to find, and always running from one to the next one could not be your best course of action.