Plus summoning new heroes would mean they would have to level up again and gather all the mats for their weapons once more. Though I don't know why the Kingdom doesn't just...keep rare monster parts around for when they need to summon heroes. Would make it a lot easier for them.
But it's interesting to know that the Heroes aren't supposed to be summoned together in one Kingdom and should be fighting across the world. So for every town they save they lose many more since they are all packed together.