I don't think that's the whole picture. You could copy an interface (and even implementation) and fork it. But often with NIH we see a reinvention of a technology without even looking at alternatives; often this ends badly (in Linux land much more often than not it seems).
The problem is that reading (and learning from) code is hard. The other problem is that there is so much bad code out there (most of my own is certainly not an exception) that it gets even harder.
And it's not about the code. Writing code is easy. It's about finding the right problems first. And then it's about finding the right abstractions. The easiest way to build a clean conceptual world is to start with a clean slate and ask yourself before introducing new code, "does this code solve a concrete problem? Do I really need it?"
Unix had simple and clear ideas, and it has many reimplementations (NIH!), and most are not that bad - are they?
