Flat-pack furniture is not simply assembled furniture in pieces. The whole product is designed around the cost of moving air, and that constraint reaches every part of it.
Freight is charged for space
Containers, lorries and warehouse racking are limited by volume long before they are limited by weight, so an assembled wardrobe pays for the empty space inside it.
A flat carton uses a fraction of that volume, which means many more units in the same container and a far lower transport cost per item delivered to a customer.
The saving compounds at every stage, because the same carton is cheaper to store, cheaper to handle and cheaper to carry up a staircase at the far end.
What the design has to give up
Panels must be flat, rectangular and consistent in thickness, which rules out shaped timber, curved sections and traditional frame construction almost entirely.
Joints have to be made by an untrained person with a supplied tool, so the design relies on cam fittings, dowels and threaded inserts rather than glued or cut joints.
Those fittings are strong in compression and weak against repeated movement, which is why flat-pack items tolerate assembly well but tolerate disassembly rather poorly.
Why the materials are what they are
Engineered boards are used because they are dimensionally stable, flat, uniform and cheap, and because they machine predictably for automated drilling of the fitting holes.
Solid timber moves with humidity, which would misalign pre-drilled holes and make assembly unreliable across the range of conditions a home might present through a year.
Surfaces are foils, laminates or veneers applied to the board, and their durability depends far more on the edge treatment than on the face of the panel.
Where the quality differences sit
Board density is the main structural variable, and it shows in weight, in how well the fittings hold and in whether shelves sag under load over time.
Back panels are the second signal. A thin sheet pinned on contributes little rigidity, while a thicker panel set into a groove braces the whole carcass against racking.
Edge banding is the third, since an exposed or thinly covered edge admits moisture into the board, which then swells and cannot be repaired once it has.
Why assembly instructions matter more than they look
Fittings are usually specific to a position, and interchanging them produces a structure that stands up initially and then loosens under entirely normal use.
Tightening sequence matters as well, because a carcass drawn square before final tightening carries load evenly and one drawn out of square never recovers.
Most reported failures of flat-pack furniture trace back to assembly and to loading beyond the design intent rather than to the components themselves.