Rush hour in a private car is a very unpleasant experience to most people as well, it doesn't help much that you get sit inside your own metal box. Cars take so much space that vast amounts of taxpayer money has been poured on infrastructure in order to try to keep traffic flowing – and we still have congestion.
if it is running near capacity, its crowded, and therefore unpleasant, which causes people to attempt to adjust their schedule to use it at other times, shifting the load
if its not running near capacity, its wasting a lot of space, and therefore more wasteful than private automobiles
in both cases the scale of the system and the centrally planned nature of the system make its response time lag behind the people using it, causing them to seek alternatives if they are able
