Because there is a point of diminishing returns with volume for the day, for example on your first set of bench you'll get a large adaptation response, on the second you might get 80%, third 60% and fourth 40% (rough numbers) but the fatigue debt is the same for each set, making it harder to recover the more volume you do.
So if you get a 240% adaptation response from doing 3 sets of bench 3x per week, you'll recover from and see better gains than if you were to do 9 sets once a week.
my explanation is kind of poor and confusing, Here's a link with some useful information http://www.powerliftingtowin.com/pow...ing-variables/