Yeah, I don't like that. It doesn't feel like making a tough choice to me. It feels like being arbitrarily limited for no reason. Nothing in any AD&D or D&D lore that I can think of explains why such a thing would happen. It's not like in, say, the Cypher system where you're limited to carrying three Cyphers because they're filled with strange energies that react badly to each other or that are dangerous to you if you carry more than that number. It's not like in the Discworld novels, where large concentrations of magic can cause the books to come alive and potentially eat you. Instead, the limit is purely for meta-reasons, because otherwise the wizard would be stronger than the fighter.