Well, you can defeat an army with just a few heroes, but only if those heroes are well-leveled, which isn't easy to achieve. It requires a lot of investment in terms of resources and time, potentially more than it would take to build an army. That's where the balance lies. So, the game is essentially saying: improve, players, level up your heroes – they are the most important thing... Although, it's frustrating when these under-leveled heroes constantly die in battles and you have to revive them again and again.