Again, because board prep isnt the main charge of pre-clinical medical education.
This may be hearsay but it is told that a "lower ranked" school back in the day adopted step-1 targeted preclinical curriculum and ended up with the highest average board scores in the country. These students subsequently entered the wards ill prepared with disasterous results, prompting the school to revert back to a more balanced curriculum.
There are a lot of things a school needs to teach besides every intermediate of kreb's cycle. Things like patient interviewing skills, bedside manners, history taking, physical exam, understanding the humanistic side of medicine. These factors play a suprisingly important role in the wards and in practice, and given the limited time schools have to educate students in their first two years, it must be balanced with stuffing students with preclinical knowledge.
Sure, you can just have a basic sciences board focused curriculum and score high on the boards, but you may end up with basic science robots that are completely inept in the wards - and your clinical grades are as important if not more important than your Step 1 in matching.