> Wow, that's shockingly low. The paper doesn't describe why --

It's because many drones, even "commercial" ones, are built using hobby parts with poor QC. If you use high end components then the reliability will be up by a two orders of magnitude, as will the price.

(Also as mentioned by others, multirotors and especially quadrotors have many single points of failure and so even with reliable parts the whole contraption isn't very reliable.)