Not, where are the human and god adults who are the parents of the demigods? Because they are around, and given their characterization, their actions are plausible. No, it's where are the older demigods?
Given the number of people that are cited as demigods in the work -- Houdini, Harriet Tubman, George Washington and so many more -- it would be plausible that somehow they managed to work off their attractiveness to monsters as they grew up, and once that's done, they forged a new career putting their skills to work. But at the very least, they would have people graduating from camp to do that, and it would get mentioned to the young campers.
"They all die in the quests" is not plausible. Not enough of a death toll in the books. Even in the last one, even if that was a typical toll, there would obviously be some survivors. And these were obviously peak years.
Now, perhaps it could be plausible that periodically, heroes do not hunt monsters because they have cleared them up, and instead fight, well, each other. Hercules, Jason, Theseus, all had to fight monsters; Agammenon, Achilles, Hector, Paris, all got to fight the Trojan War. Odysseus, of course, had to deal with monsters, but only after his ship had been swept out of the usual regions of heroes, where, perhaps, the earlier generation had not gone. That would even explain WWII, which is, in story, the fight between two sets of demigods. However, I think that would have some indications in the story. (It certainly would have encouraged them to take on careers, considering that would work better than wars to occupy them.)