They're closed to contributions because they're my personal projects. They serve a very specific purpose - my playground, my mental space.
But they're open source, because if others are curious about how they were built, I don't mind if they want to see the source.
But as soon as you allow contributions, is the project really yours anymore? Is it fair to the community that has contributed to your project, all of a sudden to be locked out from it, and forced to fork it to continue it's evolution? Why not hand it over to the community and start your own fork for personal development? I think that is the larger debate to be had.
For others, though... I don't see a reason why forking a project would be a bad thing. Yes, it fragments the community, but it also means different ideas can be tested and implemented by the people that care about them.