It may have to do with not using a car instead. Even here in the Netherlands, a car can be a status symbol in certain circles — particularly among the yuppie office crowd.

There is also the fact that this is a certain subclass of cargo bikes with a bench in the 'cargo hold' that tends to be specifically designed to hold two young children. These bikes are marketed at yuppies (in both price and design); people who can't afford one of these, or who are more pragmatic about it tend to settle for attachable seats (mounted behind the handlebars or behind the saddle on the ubiquitous rear luggage rack above the rear wheel) on a normal bicycle, so riding a special cargo bike just for the few years that the kids can't ride their own bicycle may come across as overly luxurious and soft.