Programmers get stuck at a certain amount of complexity. Newbies can write brute-force programs up to 2,000 lines or so, but beyond that it's too hard to manage. You can't keep everything in your brain. Experienced programmers have other tools -- abstraction -- to get up to 20,000 lines in a single project.
There are other complexity walls: at 200K, then 2M. There might be a hard limit of any project at 3M lines: "the growth rate seems to slow down significantly no matter how many people (hundreds) or years are involved (decades). "
The author also clearly highlights the problem of Every Line is a Potential Bug -- go read!