Obviously because there is a big cost(maintenance, wages of employees) which cannot be undertaken from many governments as a social provision. However in some countries like as in my country for some social groups such as pupils, students, unemployed, large families, the elderly, etc. the law gives them the right to pay a reduced ticket which 50% of the regular one.
There is also the assumption that most citizens will have vehicles, and choose the most efficient method of transport (time wise) because of convenience. No one with a car, is gonna wake up 4 hours early to catch 3 bus exchanges to get to work. Otherwise, there would be more people than seats on public transportation, and both the amount of transport, and the cost of transport would have to be raised to keep it together. Public transportation serves a very specific function; providing transportation to social groups which typically don't possess vehicles (students, elderly, the working poor).