It is my qualified guess that the main reasons they have kept their hollowed-out craft technology is simply because they can hide in plain sight, and these solid ships are still excellent weapons.

Since their first attempts to conquer space as a species anchored in 4% space/time with solid bodies like ours, they have evolved into becoming interdimensional beings who can also nano-travel. 

It is my qualified guess that the main reasons they have kept their hollowed-out craft technology is simply because they can hide in plain sight, and these solid ships are still excellent weapons.

 However, it is my full conviction that the major threat from them do not come from some kind of invasion where we fight in physical bodies in some huge Battle of Armageddon. Albeit, this may be a part of it, the major threat is going to come from the nano world.