Anyone know why Subway Builder uses a gravity model, when the US census publishes a complete origin-destination matrix?

You don't have to guess to simulate where everyone works - the census literally publishes this information in the LODES dataset. I feel like this could increase the accuracy of simulations, e.g. accurately reflect that some lower-income neighborhoods are less likely to work in a financial district (compared to higher-income neighborhoods of a similar distance away), etc.