It's not an issue of hoarding knowledge it's just the fact very few people play with and toy with the official sdks.
If you read the docs that come with them it's trivial to install them, get set up with tools, compile samples then boot those samples on a CDR or emulator.
I'm not exactly sure what the issue is.
Furthermore from what I've seen, the people that want to jump into making something for dreamcast go find those sdks, install them, don't read the docs, don't know anything about Dreamcast, then bombard with questions not even relating to the sdk. Instead they have very little or no knowledge of using c/c++ and instead ask questions relating to that. Which means even if they are handheld with basic C questions, once they get into making something / modifying the samples they will hit insurmountable walls.