There's a highway for planes in the sky and it's super complicated

One of the things that stuck out to me the most from this really interesting video is the fact that planes still go "dark" from any sort of communication with towers or airports over long distances. I mean, don't we live in an age where we can solve this issue? Isn't this precisely the reason why planes like MH370 have not been found? I find it both alarming and fascinating that we haven't invented some sort of system for this yet. What are the real hurdles here?