You're comparing brand new cargo bikes to used cars. The real comparision would be a new cargo bike (2-5k) vs a new car (>40k).

You're making the assumption that the cargo bike seats only one person, when a parent could easily seat two children on a cargo bike.

You're making the assumption that everyone would be using the cargo bike as their primary day-to-day transportation, when the cargo bike is mostly useful for either transporting young children, or when transporting cargo. At other times a more reasonable choice would be to use a regular bicycle.

You're making the assumption that cargo bikes take up as much space as cars which is just downright false.