One problem is regular trips, such as commuting and shopping, where most people need to regularly go from residential areas to office areas or to commercial areas. Here, the vast majority of people, all over the world and with few local exceptions, use public transport (trains, subways, trams, buses, etc). The fact that you have to walk a little bit to the station and from the station to your destination is not a major concern, compared to the cost and the time difference VS filled city streets. If all the people commuting, say, by subway in Tokyo were each in a robotaxi, you'd need most of the day just to finish the morning commute.

Then there are more independent trips, where your schedule is not aligned with public transport, or where there is some urgency, or where your destination is very far away from a station, or where you have very heavy bags etc. For those cases, a car is of course the ideal, so lots of people, even many of those that commute daily by public transport in cities, also own a car.