Because it will be so hard to do so. Hybe’s system is not your typical Kpop company setup where management and contracts fall under the same company. They have two authorities, so the idols most likely have two separate contracts: one for their label and one for the main label, or their contract is solely under Hybe as the parent company.
Look at 50/50. The girls have evidence of being mistreated to some extent, but as Attrakt’s ceo, Jhj, blamed everything on The Givers/ASI as the management team, the 50/50 members were unable to claim anything directly against their company. The same thing could happen in Nj and Hybe situation. Hybe will play the 'not us' card, blaming everything on the new Ador ceo, or even Mhj and Ador management teams. Remember how they already blaming Mhj behind the delay of Nj album and tour? Proving that Hybe mistreated them would be difficult because the ones directly dealing with them are the new ceo who will be used as a shield for Hybe and the one that will take all the blame in the court, Mhj, and Ador staff, not Hybe higher-ups or staff—especially since Ador has its own separate team from the main company. It's much easier for Hybe to denied everything and accused the sublabels as not competent instead by themselves.
If the girls want to terminate their contract, they need to plan carefully and make sure they have everything in place to defeat Hybe. It will also be a long journey, potentially taking months or even years to resolve. While the process is happening, Hybe will likely keep them in the 'dungeon' with no activities. Remember, it took almost 3 years for Jyj to finally terminate his contract with Sm. Hybe will also most likely bombard them with lawsuits to wear them down emotionally and financially. By the time everything is settled, the girls may have already lost their momentum.
So yeah, it's not as easy as 'just terminate you contract if you dislike Hybe and want Mhj back'. It's never that easy. Fighting for Mhj to remain as the ceo at Ador is definitely easier for them to done and, honestly, the best choice for their own sustainability and careers. They get their original Ceo back, and they avoid having to go through lengthy direct legal battle with Hybe with no guarantee they will free from Hybe.