What if they became rich by stealing other peoples rightful inheritance?
(What proportion of the King's wealth derives from the time before Magna Carta and the concept of legal restraint on the Crown? Until then it was effectively impossible for anything the monarch did, including the most blatant theft and murder, to be deemed illegal)
In re "stealing other peoples rightful inheritance", as a side comment, the Crown (but not the King) is the beneficiary of inheritances for which an owner cannot be found.
So, theft under color of law.