I did wonder that. In areas well served by public transport, it can be a viable option, but for a lot of people cars are a necessity. I have friends who live within walking distance of shops and their work, so they never needed a car. Even on longer trips the train and bus services are adequate for them. On the other end, there is a housing shortage. A common complaint is that where new houses are built are not well served by facilities. It is almost a requirement to have a car to use such housing. For my commute there is no practical public transport option that is both affordable and would take a reasonable amount of time.
I was considered crazy by some when I was planning my US trip, wondering if I really need to rent a car while I was there
From the ideal hotel it would have taken around 20 minutes walk to get to work, and downtown was also a short walk away. In the end I had to stay at a hotel some miles out which made the decision for me. If I wanted to travel, there's always Uber, which I used anyway e.g. if I wanted some drinks. In a more built up area, maybe I could have got away with walking. At least most places had free parking otherwise it would be hell.