The main problem is that the cul-de-sac neighborhoods are often poorly designed. Two houses which are a stone's throw separated can be a 30-minute walk away, simply because nobody bothered to add pedestrian / cyclist shortcuts. The entire neighborhood is designed around cars, so any kid walking down the street is forced to interact with cars.

Also, there's no reason why a cul-de-sac neighborhood can't be dense and walkable. Street layout and density are only tangentially related.