Why is USB type C connector hollow?
USB micro-B is known for its high mate/demate cycle specification relative to the other USB connector types. It achieves this by having the sprung contacts in the cable, rather than in the device. This way, only the cable is subject to deformation, and can be replaced if it becomes unreliable. The contacts in the device are fixed and so do not wear or deform as readily.

The type C connector is similar to the micro-B, and was presumably designed with the same principle in mind, because it is also rated for 10,000 cycles. To have sprung contacts in the cable without any sort of shroud will mean that they protrude from the body of the connector. This could result in their catching on something and bending or breaking off.

The Apple Lightning connector's durability is discussed in US9004960. I could not find any mention of how many mate/demate cycles it is rated for, apart from "sometimes thousands". This sounds like it might be less than 10,000.

Note that the mini-DisplayPort connector (used for Intel's Thunderbolt in past iterations, now to be superseded by the USB type C connector) is designed along the same lines as USB type C. It seems to be an established design principle for this sort of application.