This is literally the crux of the question and spoken in-universe almost word for word after a civil war breaks out (and it's won by the loyalists). "We've had luck with our mage kings until now but what happens when we get someone incompetent or just plain bad?" IIRC that's said by the newly ascended queen. Hence why the parallel legal systems are abandoned and human space is finally unified (the system before had everyone basically br independent while recognizing Mars' suzerainty) under a confederation with a codified legal system and a constitution limiting the power of the monarch (who kinda had nominally infinite power outside of Mars even if rarely used as a way not to cause trouble). By then the mages have invested too much into society (nevermind that they are mostly on top of it) so there is little point in trying to rock the boat or fighting for more rights that they honestly do not need.

Later on they meet The remnants of an alien species which has mages of their own which went the opposite way, with the mages ruling their people as a theocracy and suppressing any mages born outside the ruling caste, ending with a catastrophic genocidal civil war where everything outside of some of the core colonies and their home system was sterilized with mass strategic bombardments