perhaps because it's something divine unique to royal fam?
It's a tradition since the reign of first emperor. Rykell will explain it in ep.54
the first convenant was the proof that the heroes and transcendents trust the first emperor (and the 1st emperor was a wise ruler). It's became strayed in her descendants' era because the next emperors used the convenant to limit the archmages and swordmasters movement.
the heroes and transcendents who chained in this generation are : Rykell (Archmage of the North), Raul (Duke of the West), Margot (Archmage of the East. she is the disciple of Galatea/Maybelle's mother), Von Kyle (Hero of Tephtol War).
Rykell actually CAN unchain himself because as the strongest Archmage among the 3, he can break it but still got hefty penalty (bellerophon will imply that it will injure Rykell permanently),but he will do it if Maybelle is in danger. Margot lost her human body thank to Emperor's black magic, and Raul can't rebel because his nephew is confined in the palace.
Isn't their any way without breaking it forcefully?
yes. there is.
kick the current emperor and annul the convenant.
why did the imperial family chain mabel's father?? and i think it was mentioned that he was powerful so uh first: how was he chained if he is powerful, second: why can't he unchain himself?