Open hardware tends to take a little bit of a specifications penalty at the design stage, because usually the most well-known SoCs are also years old, or based on very well-known architectures. There are some counter-examples to this, but it tends to involve working closely with an SoC vendor, which is rare. They also have to include the cost of developing at least some OS to ship on it, since not everybody wants to buy a fancy development board. Then, while they develop that software further and keep it updated, they kind of have to freeze that hardware design and specs in place so they aren't trying to hit multiple moving targets all at the same time.

It's not my taste either, I'm kind of on your side, but those are the reasons. While it's fun to develop hardware that's as open as possible, I think it's more practical and fun to pick up old stuff and bring it up to speed with Linux. That has no quality standard, however - you can find some gems, but you can also find some barely functional lemons, and it's hard to know which you have without doing some research beforehand and really familiarizing yourself with things like which SoC vendors and precise SoCs have the best support from Linux. That's why I understand some people wanting to buy hardware that is "open design" and open source but has strong support from a vendor in terms of software updates. That has a much higher quality standard.