Why are sofas and beds constructed so differently?

perhaps you need a better matress if you think sofas are perfectly comfy to sleep on. Effects of sleeping on a sofa may not be immediate but your back will hate you in 10-15 years. If you really need sofa to dobule down as a bed, there are sofa beds which combine both functions. You make them into bed at night. But most people prefer to have a separate bed. Not sure about you but I'd hate to sleep in the same place people sat with their outdoor clothes on. It works if you have a very small flat but since most people prefer to have a separate bedroom, having a sofa doubling as a bed would be annoying. Sofas are also in living area so, unless you live alone, quite uncomfortable to sleep on with everyone having access to it.