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I am not sure what you consider to be a low number of private cars or pedestrians, so it is difficult to calibrate your query about the numbers being low. In Simutrans-Extended, private cars are not a purely cosmetic feature: they represent actual private car journeys made by simulated passengers. The number of journeys will be affected by the era (if you have the timeline enabled), how well connected that the town is by road to other towns or out of town industries and attractions, the proportion of wealthy people living in the town (wealthy people are more likely to have access to a car), and how good that the public transport is (passengers may prefer to use public transport instead of using their car).
In 1815, you will not see many private cars, as only wealthy people, of whom there are very few, can afford their own private horse carriages. As time passes, and especially as the 20th century comes and motor cars become available, the number of private cars will increase considerably, especially if your towns are connected to each other by road.
As for pedestrians, again, in Simutrans-Extended pedestrians are generated when actual walking journeys are made by passengers. This includes walking to and from public transport, as well as a complete walking journey from origin to destination.
Both pedestrians and private cars will be more numerous in a game in which there are more passenger journeys made by these forms of transport. Look at the "walked" and "traffic" graphs in the city information windows to see what the numbers of people using these forms of transport are.
