Well sure, when you put it that way it sounds simple! I was 12 or so, and I spent a few nights trying to figure out the "plain data file" before shrugging and going back to playing X-Com.
Well, reverse engineering binary data is hard and laborious, and probably incomprehensible to almost any 12-year-old (and the majority of adults).
(I'm assuming they were binary. I really can't remember what they were.)
Gotta be honest, I installed the game tonight under DOSBox thinking I would give thirty-something, seasoned engineer me another crack at decoding the file format. But then, those UFOs kept landing and I mean, somebody had to do something about it. And then all of a sudden my kids were like "daddy where are you" and the file format (and UFOs) once again had to wait for another day...