[Grishnakh]

Twin bets aren't adult-sized, they're for kids. And full-size beds are basically obsolete, or just for short people. They're simply too short, given that people are taller now than they were 50 years ago. Queen size is now the minimum normal adult size bed in the US; that's why you see them marketed as the "average-size bed for two people".

If you're single and really want to save space, you can get "long twin" mattresses, which is exactly half the size of a king mattress, and also the size of the foundation used under a king mattress (x2). But most single people these days just get a queen size, because it's long enough for most people while providing plenty of space to spread out, but also gives you sufficient space in case someone spends the night with you... Limiting yourself to a (long) twin size makes it rather difficult to have a guest for the evening unless you're really thin, which Americans generally aren't these days.