So there were sleeper planes, but the space requirement for beds made them uneconomical (except in first class).

The absence of "sleeper berths" in standard economy is purely a matter of aerospace economics and weight. To provide a bed, an airline would have to remove roughly two or three regular seats, meaning they would need to charge triple the price for that space to remain profitable. Historically, planes in the 1930s were slower (taking 16 hours for transcontinental flights) and featured berths because they carried fewer passengers. In 2026, modern jets are so fast that most flights are under 8 to 10 hours, making "upright" seating more efficient for mass transport. Furthermore, safety regulations require passengers to be in a seated, belted position for takeoff and landing; providing both a seat and a bed would effectively double the footprint and weight per passenger. While airlines like Air New Zealand have introduced the Skynest (bookable sleep pods), these are supplemental and temporary rather than a standard configuration, as "seat density" remains the primary driver of airline revenue.