Because we want the best 5 on the field.
For example using like madden ratings, what if:
You have 5 OTs that are rated 74, 77, 79, 85, 86
You have 1 OG that is rated 86
You have 1 OC that is rated 67
Who are you starting and where? Picking the 5 best pure players is easy. But which of those 5...