There are a lot of people going the same place at the same time actually. Think about factories, schools, offices, and so on. With a working public transportation network you can make people have a better transportation that driving. Not all case and people are covered and for those people using a car or a mixed mode could be an option too. This is going to decrease the overall traffic, so people that must use a car have an advantage.
There is a mall near me that was built initially with a car centric idea: near the motorway exit ramps and with a giant parking lot: they subsidized two bus routes to have a stop in front of the main entrance of the mall, and hava a bur every 15 minutes, and it worked, they got more teen without diving license and older people that lost their license, but also people that have a car but they don't want to use it to only visit the mall to buy few items or to eat there.