That would be a waste of money. Building transit lines without people to ride them means that they reach their end of useful life from old age without serving enough people to be worth them. It would be environmentally sound to build another freeway - at least that somewhat reduces congestion caused pollution (until the city grows and you need a new one).

Build high density around a bus stop, and some of the people will take the bus. Those additional riders mean the bus is making more money and can afford to add more routes. More routes means the bus is useful to more people who start riding the bus... Eventually you have enough people on the bus to switch to a better transit mode. The bus rider levels also show where people want to go which guides where you put the trains instead of a guess and prayer. But this all requires enough density to start the cycle in the first place.