Because what if they're not rational? No one else seems to be bothered when we go tromping through caves. Not even Gladio, and he's huge. You'd think the biggest guy in the group wouldn't want to risk getting stuck in some narrow little passage, right? But no! I'm the smallest, and am one step away from a panic attack because the ceiling is low. And I don't even have to duck!
[Unlike Ignis and Gladio, who towered over him and Noctis, and never seemed bothered by anything. Add in fighting in a tiny space, trying to shoot quick little demons and not his friends, and well. Maybe it was rational, but maybe he was just a wimp.]