Why is open hardware so rare?
Everyone can edit source code at home, very few people have a chip fabrication plant to knock out a couple of custom chips. Bytes are free to create and distribute, materials are not.

There's also the issue that source code is portable, and although CAD files etc. are sort of portable, there's a lot more overhead &amp; errors &amp; setup cost wasted materials.

3D printing crosses some of the boundaries, perhaps a bit of effort could do the same for the (much older) technology of machining, both parts &amp; PCB's.

Edited to add: re-reading the question, and perhaps the intent of the question relating to FPGA's, I would say that they're currently still a bit of a dark art to many, and just not on the radar of most people. The entry barrier is quite high, in terms of effort, understanding, and tools.