I think it's because of the lack of feedback. Humans also can't do much without feedback. E.g. I doubt most people could write 100 lines of code that works first time without even compiling it once.
It was much more inefficient, because it's easier to find bugs after compiling or running the code. But it is perfectly possible.
Sorry when have you ever had to write code without being able to compile it? Tools have never been that limited.