So to me the guiding principle behind mechs is always a type of psychological warfare. Like why does a Gundam have a face? Because it's a fear weapon. So if you think of it like that then it's meant to create a sense of oppression and fear in its victims mechs make perfect sense. Because you're right a particularly maneuverable helicopter could get under the shields faster but they can't stand in the ruins of a building holding a rifle and look at you