This is because the data (language in general, child directed speech in particular here) are naturally distributed such that particular nouns that refer to animates tend to be agents, and tend to appear as first nouns, and those that refer to inanimates tend to be non-agents and second nouns.
a learner representing sentence information in terms of words only succeeds with full-distribution 'A gorps B' test sentences even with the simplest animacy feedback (Feedback 1); the A and B nouns in these test sentences reproduce the learned distribution.