One important reason is to have students "get their hands dirty". There is a skill to performing (and observing) experiments which is different than just reading in books, and a skill in working with biological organisms that is different from chemistry experiments. If every pair of lab partners dissects a frog there is sure to be some organ that some teams don't find, and not every frog will look the same. Experiencing this is an important pedagogical activity.
Because it would be uncomfortable the other way around.