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Because hardware is, well, hard. There is a huge upfront investment that isn't even remotely comparable to the amount of money you can spend on software development, and equally huge incentives for third parties to undercut you by taking your designs, manufacturing them for cheap and offloading support onto you (as already pointed out Arduino is a great example of this happening in real life). Even if everything is open source you have to build an entire business and marketing department around selling the hardware, while with pure software you can just put it up on GitHub and call it a day.

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.