If you use any of the free document generators out there, you are left to your own to figure out what to do with the generated documents. It's not rocket science - you can definitely figure it out, with enough research. But it will take you a long time and it's cumbersome - consequently, you are highly likely to mess it up in some way or another.

We started Clerky exactly to solve this problem - as startup lawyers, every time we saw someone try to save money by doing paperwork on their own, there was some sort of issue with it. Yet we could definitely see why they wanted to do it on their own. So we built software to make it impossible to mess things up (as long as the information entered is correct).

It's hard to say why people reliably mess things up. I think part of it is that it requires of a level of attention to mundane details that is not natural to most of humanity. Another part of it is that it really helps to know some of the reasons behind why those mundane details are important - which requires some knowledge of how corporations work, contract law, etc. And it's one of those things where you don't know what you don't know.