Short answer- to save you, the customer, time and money. Some companies only sell their covers and inserts separately and charge more for each. We don't want to nickle and dime you so we have ours bundled because that's how the diaper works, you need both the cover and inserts.
For more details, here is our very own Mohammed (one of the great founding fathers of Lil Helper) explaining why we don't have just the covers available on our website. >