Yeah but at night there's significantly less traffic, so it's ok to stop the trains and rely solely on the bus network because it has enough capacity to handle night traffic.

Where I live is an example of that: during weekdays the light rail closes IIRC between midnight and 6, they only keep it running on weekends because of all the people going out at night (the metro always runs but that's fully automated). Just because the bus system isn't enough to handle a city's peak load, it's usually fine for handling the load on periods where the load is the smallest.