Tou can buy parts individually, but its not often that someone buys parts individually unless something breaks. The poont is it minimizes skus (two case designs (13", 15" built around repairability) designs dont go to waste if one thing breaks. They also open their own marketplace for users to resell their used parts as parts can freely be used in other builds.
They dont have much to hide, the point is that they claim theirselves who is pro repair, minimal sku and choose your own ports is not sustainable, then any laptop manufacturer who designs laptops that are going to be desposed if a single part breaks is going to have it worse, over several different Skus with different case/mobo/layout and part selections. As they would heavily fail in the 2nd R (reuse) category.