Well, this may be one reason: It's possible the game stores the amount of units as a 16 (?) bit integer. This would prevent it from being any larger than about 32k... The bug is present in many other games: "It's a programming thing." Congratulations for having perhaps the biggest horde game in all history.

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"The warrior who does not know his business is like a cat that does not know the way of ratting."
- Tsukuhara Bokuden