Look at almost any traditionally embroidered garment and the stitches avoid the middle of the fabric. They run along the collar, down the front opening, around the cuff, along a hem, or in a narrow band over a joining seam. It is tempting to call that a taste for borders. It is closer to a rule about materials: a densely stitched patch is stiffer, heavier and less resistant to rubbing than the cloth it sits on, so it belongs where the garment is already strongest.
A stitch is stiffer than the cloth
Every pass of thread locks a small number of warp and weft yarns together. One line of stitches does little; hundreds of them close together stop those yarns from sliding over each other, and the area stops draping like fabric and starts behaving like a thin plate. Put such a plate in the middle of a back panel and it will not follow the bend of the spine. The surrounding cloth will, so the panel puckers around a stubborn island.
Run the same stitches as a narrow band along a collar or a seam and the stiffness becomes useful. It acts as a stay, helping an edge hold a straight line instead of rolling over. A collar that stays flat is built on exactly this idea, only with an interlining rather than thread. In both cases a local increase in stiffness is placed where the garment wants to stay rigid anyway.
Seams also have more material to hold a needle. A folded edge, a facing or a binding gives two or three layers, so the stitches do not have to pass clean through a single thickness and show a messy reverse. The thread is buried in a sandwich, protected from the inside of the garment, and the whole band survives repeated washing far better than the same work done on open cloth.
The seam is the skeleton of the garment
Structural lines are where a garment is joined, where its shape changes, and where loads are transferred. They are also the lines along which it deforms least when the body moves. A pattern laid along those lines travels with the skeleton; a pattern laid across the shoulder or the inside of the elbow is stretched and compressed every time the arm is raised, and the thread, being less elastic than the weave, is the first thing to loosen.
This is the other half of the map of where clothes wear out. Friction concentrates at collars, cuffs, plackets and pocket edges; those are the places that need reinforcement, and they are the places a band of stitching happens to sit. A bound edge began as a way of stopping a raw edge from fraying. Covering it with stitches turned a repair into the most visible part of the garment.
A frog fastener makes the same point in miniature. It is made from a narrow bias-cut strip of the garment's own cloth, sewn along the opening, and it is at once a closure, a binding and a decorated line. Nothing is added to the middle of a panel; all the extra structure lies on the edge that has to be firm anyway.
The exception that confirms it
There are garments with large embroidered squares on the chest or back. Almost always the motif was worked on a separate piece of cloth and only then stitched onto the garment. That is a crucial difference: the stiff panel is an applied part with its own boundary, and if it wears out or goes out of use it can be removed and replaced. The drape of the garment underneath was never asked to absorb it.
The same logic shows up far from clothing. Ribs inside a plastic box are placed along the paths where the load travels, not spread evenly over the whole wall. Stiffness is cheap when it is put where stiffness is wanted, and expensive when it is put everywhere. Embroidery is a stiffener that happens to be beautiful, and a good maker treats it as one.
A quick check needs no special equipment. Take an embroidered edge and fold it once, then fold a plain area of the same garment beside it. The plain cloth creases softly and recovers; the stitched band forms a distinct ridge and holds it. That ridge is why the decoration ended up on the seam in the first place — not because the middle of the cloth was too plain, but because it had to keep moving.