Ultimately, RAW formats aren't that complex, and camera firmware is mostly developed in countries that don't have strong open source software traditions.
Look at the decoders for each format that darktable supports here: https://github.com/darktable-org/rawspeed/tree/develop/src/l...
It's some binary parsing, reading metadata, maybe doing some decompression-- a thousand lines of C++ on average for each format. These aren't complex codecs like HEVC and only reach JPEG complexity by embedding them as thumbnails!
Cameras absolutely could emit DNG instead, but that would require more development friction: coordination (with Adobe), potentially a language barrier, and potentially making it harder to do experimental features.
Photographers rarely care, so it doesn't appreciably impact sales. Raw processing software packages have generally good support available soon after new cameras are released.
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