It strikes me that open source hardware should be more common. It's surely much easier to monetise than open source software: you just sell the hardware, because noone wants to build one themselves. Why isn't it?
Not to mention that in this day and age every piece of hardware has software at its core, so open source hardware does not save you from also writing the code that runs on it. If anything developing open source firmware is actually harder, because most chip vendors expect your product to be closed source and want you to sign countless NDAs before you can access a datasheet or even just buy the chips. You are restricted to using older and/or more expensive parts whose documentation is freely available; it's the exact opposite of the software world, where the latest and greatest is one npm install away.