Because a wound doesn’t close the way a door closes. It fills.
The surface part comes first. When a wound bleeds, the blood clots dry and form a scab, which protects the tissue underneath from germs. The scab is a lid, and it arrives early — within the first few days, while the area is still slightly swollen, red or pink, and tender.
What happens underneath takes much longer. Over roughly the next three weeks, blood vessels repair and the wound starts to fill in with new tissue, called granulation tissue, with new skin forming over it. As the wound heals, the edges pull inward and the wound gets smaller.
So a scab on top and an open space beneath it are not a contradiction. They are two parts of the same process running at different speeds.
Surgical wounds that are left open on purpose show the same thing from another angle. Such a wound heals from the inside out, and the dressing does a specific job: it absorbs drainage and keeps the skin from closing over the wound before the tissue underneath fills in. Clinicians actively work to keep the surface from closing too early, because a wound that is sealed on top before it has filled underneath has a space left inside it.
That is also why you shouldn’t pick at or scratch the scab. The scab is doing a job, and what is under it is not finished.
What this page can’t tell you is whether your own wound is filling in as it should. Depth is one of the things measured at an exam. Dead tissue over a wound makes it hard to tell how deep the wound is — which is exactly the judgment a clinician is making when they look at it.