I would imagine that it's because some fools back in the days of waterfall when all we had to program with were sticks and clay tablets and you had to throw your software into an oven on 150 C for 30 minutes before running (true story) thought that software development was as predictable as cooking. After all, you know your starting point and you know exactly what you want to create as the end product. So you can plan it out. And once that's done and the project starts you won't ever have to deviate from the plan to change things, rework old features or fix bugs. Nope.
The plan, once carved and baked is sacred. And there is simply no reason why you would ever deviate from it. And thus, obviously, it makes perfect sense you should start your version numbers at 0.0 and end them at 1.0. Right? Right?
Yea... And than reality happened. Unfortunately no matter what the universe says or does the moment that clashes with human kinds expectations they have a remarkable way of being convinced it's all of reality that's wrong. And so the 0 - 1 versioning myth persists and ensnares developer after developer (including big corporations, nobody is safe) time and time again.
Worse yet, it's something that has gotten so ingrained in our popular psyche that we as buyers expect it. *sigh*
It's one of those things that sound just logical enough on first glance that you accept them as true unless you know better. And by know better I mean think about it for 3.2 seconds or more. But unfortunately in my personal experience those 3.2 seconds typically happen after you start adding ....9 at the 4th or 5th decimal point.