It's in part laziness - reading and understanding someone else's code takes a lot of focused effort. But it's also a matter of gratification - writing code is far more satisfying (IMO) than reading and comprehending others'. Most developers I know - and probably myself included - are of the "I get paid to write code" mentality, and often not really being mindful of whether what they write now will still be understandable in 5 - 10 years time. Sure, they will follow best practices, clean coding and all that stuff, but what about (high-level) documentation? Revisiting the user stories from which the code was born? Writing good commit messages and pull request descriptions? These all seem secondary to the code itself, which I guess is true to a degree, but all that other stuff is important too.
Getting into a new project is hard. Starting from scratch and writing your own is easy. Writing code and other stuff so that others getting into your project is easier is hard (as is allowing others into your domain and code for that matter)