>>72263069They own rights to their music if it was originally funded by them and later not sold/given to Cover. Projects funded by Cover (e.g. song Rebellion by Advent girls) and various 3rd parties (e.g., HoneyWorks) are not owned by the talents. Someone like Mori could quit Hololive and release/perform all of her pre-UMG songs as another character (or RM), but not as Calliope Mori.
I was always under the impression that organs who left Nijisanji due to music-related rights were specifically unhappy because they, unlike holomem, never get to own their shit. Because of that, there are barely any original songs that were not funded by 3rd parties or Anycolor. In this regard Cover are actually really fair to their talents.
As to why anyone would sell their rights...Let's be honest here, if your song got 1-3M views in half a year, it's not exactly a hit, and even if you were later use it on your RM/PL account, you'd need to create another MV, which is unlikely to ever pay off. By selling their rights, they can essentially recuperate a good chunk of the money they've spent and reinvest it into another song/project. If I was Suisei or Calliope, I'd probably wouldn't sell my rights, but for every other talent that has no real music goals, it's the best course of action.