The major problem with all the discoveries and rewards is that the kingdom just cant choose to not keep everything, even if MC is a good person he would have enough power and resources to make his own kingdom. So they MUST strip everything from MC but also have to compensate him, or else he could deny to not get paid and keep the treasures.
Yeah yeah he wouldnt do that, but the system in place must account for someone smarter than MC, who would happen to achieve the same and choose to keep the treasures of their findings. Remember that this was only one dungeon from the ancient times and they already have at least another two airships, which means more dungeons like that could exist.