For example: MinnowBoard. Open, right? Well, the layout is locked up in the format of a very expensive CAD package. Completely inaccessible.
You do know there's a free viewer for the board file, and that if you want to edit it, Allegro is available in a range of options with dramatically varying price tags...? The version I use - which includes OrCAD Capture as well - didn't cost much more than the laptop I run it on. It's branded as OrCAD PCB Designer, but it's the exact same binary. If I double click a .BRD file, the splash screen that pops up even says Allegro on it.
I'm afraid I don't understand why anyone would describe it as 'completely inaccessible', when it's trivial to request and install the free viewer from Cadence.
I suspect the problem is really just that the hobby market is very inward looking... everything has to be Eagle, or maybe KiCad - but a professional PCB designer won't be using those tools, ever.