Because the good old days when they were seventeen and the government was funding giant death robots to genocide or kidnap them and their teacher's old pal was training teenagers as terrorists and conquering peaceful South American nations and Savage Land villages for slave labor were just so light and giddy for them, in-world.
The stories were told with a lighter tone, but in-world, the experience of, say, the first three years of X-Men stories, wouldn't have been hugely unthreatening for Cyclops or Marvel Girl. Genocide robots tend to do that.