Probably depends on the crime in the city and location. Here (Vienna, Austria) a lot of people just store them outside. We store ours on one of the parking spots we have (since we only have one car) together with some of our other stuff. They do take space, but I'm not sure if that is the main challenge with cargo bikes.
Personally I think it comes down to cost and availability. They cost around 5000 Euro if not more, and they take sometimes months to be delivered.
They don't. You need to be rich, then you keep it chained near your front door or in backyard or wherever. You can't put them into the bike shed and people who have them also have a car, so parking spot is out too.
