Furniture sold in a flat box is designed backwards from the box. The dimensions of the carton and the weight one person can carry constrain the product long before anyone considers how it looks.
Shipping volume drives the whole design
Furniture is bulky and light, so the cost of moving it is set by the space it occupies rather than by its weight.
A design that packs into a slim rectangle costs a fraction of the same object shipped assembled, and the saving repeats at every stage from factory to warehouse to car boot.
Designers therefore work to a target pack size, and a shape that would waste space inside the carton is revised until it fits efficiently.
Panels replace joinery
Traditional furniture uses shaped joints cut into solid timber, which requires skill, glue and time to assemble.
Flat-pack construction uses flat panels of engineered board joined by mechanical fittings, most commonly a cam lock that draws two panels together as it turns.
That fitting can be operated with one simple tool, holds without adhesive, and tolerates the slight imprecision an untrained assembler introduces.
The board is chosen for stability
Particleboard and fibreboard are used because they are dimensionally stable, cut cleanly, and do not warp with humidity the way solid wood does.
Stability matters more than strength here, since a panel that moves even slightly will misalign the holes drilled for the fittings and the piece will not assemble.
The trade-off is that the board holds a screw poorly once the fitting has been removed and refitted, which is why repeated dismantling weakens these pieces.
Assembly order is part of the product
An instruction sheet encodes a sequence that must work in one direction, since a step that requires undoing earlier work would defeat the design.
Fixings are differentiated by shape and size so that a wrong part will not fit, which removes the need for the assembler to read carefully.
Instructions are drawn rather than written largely because the same sheet then serves every market without translation.
Rigidity comes from the whole box
An assembled unit gets much of its stiffness from the thin backing panel pinned across the rear, which prevents the rectangle from racking into a parallelogram.
Leaving that panel off, or fixing it at only a few points, is the most common reason a unit feels unstable despite every joint being tight.
The same logic explains the wall-fixing bracket supplied with tall items, since a light structure with a high centre of gravity depends on the wall for stability.