The problem is that
- YA novels feature young characters;
- Good novels have the characters carry out crucial roles in solving the main problem of the work;
- When a young person serves a crucial role in the fighting of a war, resisting a conspiracy, or working with a disaster, the event is normally a NASTY TERRIBLE THING -- even more so when the people dealing with it are adults.
- YA novels on the hold don't want to be more nasty and terrible than adult works on the same topic -- on the contrary.
Not always. The older sorts of young characters work it more plausibly, especially if they are old enough to be a soldier, etc in real life. Sometimes it is possible to finesse it by having the character play a crucial role behind the scenes, in a certain amount of safety. But -- I've read a number of works with a certain degree of unreality to them because of this.