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Isn't the main problem that most of them are proprietary, so they can't be shipped automatically if you want to avoid shipping a distro with proprietary software?
The proprietary stuff is shipped as "firmware" (even though that's not always the case) allongside the distro's kernel. My best guess is that some distro out there (Ubuntu most probably) has obtained permission from a bunch of manufacturers to ship this "firmware" allongside it's kernel. The rest of the distro's are just riding this train, repackaging the firmware packages (if they can do it and redistribute it, why can't we 🤷).
I might be mistaken, but this is the only thing that makes sense to me. Maybe it's a semi-coordinated joint effor as well, like someone obtains permission to share firmware, writes to a bunch of maintainers and devs that "this and this" binary blob is free for redistribution and it gets picked up by most popular distros out there.