: Marketing - who buys a double? - mostly people who want to paddle with
: someone who is afreaid of the water or reluctant, families with children
: or rental orginazations who cater to people who never paddled before and
: have 0 paddling skills. Wide and stable makes them feel good and plenty of
: volume to pack all kinds of things for camping excursions.

Another aspect of the design is buoyancy vs width vs length. Carrying two people and their gear requires about twice the displacement of a single. You gain this displacement by adding volume. This can be done by adding length or width. Keeping the boat narrow requires adding substantial length and the boat gets impractical for many people. My narrow double is over 25' long. It needs a long storage space. You can keep the length within reasonable limits by making the boat wider. While my narrow double would work as a shorter boat, it would loose gear carrying capacity.

There is also the factor that sitting with someone else can make the boat feel unstable because the partner may move in unexpected directions. A little additional stability is usually welcome.

But in the end, the market demand for doubles usually wants a more relaxed, comfortable boat that makes wide work well.