While the source may be open, and it is possible to extend an Arduino s/w release with new core support,
the process of getting things into the official release is still very closed.
Yes it is frustrating at times. Several years back I got a 644p based board and installed the files needed to make the IDE work with it (around version 11 or 12 I think), but every time there was a new IDE release I would have to redo the mods and after a while I just stopped bothering. I understand the newer versions of the IDE make that effort easier but I keep misplacing the links for the detailed information steps required for actually adding a new board to the IDE.
But I can also kind of see their position too. If they indeed provided direct support to any and all 3rd party 'arduino' boards using different processors or clock speeds with their 'official' offerings, would they then not end up 'owning' all the support issues, bug reports, request for minor changes, etc, they included with the official release including the bootloaders? Sounds like a support headache they would probably like to avoid.
Lefty