Looking at this Wikipedia article, a huge portion of kit lenses start with 18mm
18-55 18-70 18-135 18-200
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It's mostly because the standard zoom kit lens for film cameras would start at 28mm. This is the widest you can typically go before hitting the more expensive exotic glass.
The APS-C field-of-view equivalent for what 28mm on full frame looks like is 18mm. So that's what most kit lenses for APS-C sensor cameras use. You'll note further down that list that for four-thirds gear, the kit lens starts at 14mm. Four-thirds has a 2x crop factor, so a 14mm lens also yields the same equivalence as a 28mm on full-frame. And the kit lens on the Nikon 1 cameras, which have a 2.7x crop factor is 10mm, yielding very close to a 28mm FoV equivalence.
There's nothing magical about the number, just that it's the easiest wide angle to develop a low-cost zoom lens for.
By tradition we first fit cameras with a “normal” lens. Such a lash-up delivers an angle of view of about 45° when the camera is held in the landscape (horizontal) positon. This “normal” view is a attempt to duplicate how we humans see the world.
The 45° angle of view is achieved when we mount a lens with a focal length about equal to the diagonal (corner to corner) measure of the camera’s fame dimensions. In your case, the frame measures approximately 16mm height by 24mm length. This size is known as Advanced Photo System (APS-C). The –C translates to classic format which is, the length of the frame is 1 ½ times the height.
Now the corner to corner measure of the APS is about 30mm. In other words, if we mount a lens with a 30mm focal length on an APS format, this lash-up delivers a “normal” angle of view.
If we mount a lens that is twice “normal” (60mm) or longer we are now in the realm of telephoto. If we mount a lens about 70% of “normal” or shorter, we are now in wide-angle territory. This would be 20mm or shorter. In other words, a 20mm lens on your camera marks the beginning of the range of wide-angle.
As we mount shorter and shorter lenses on cameras we begin to encounter some technical difficulties. The first of which is, short lenses sit somewhat close to the imaging chip (or film). This is a problem because we would like some room to attach a swinging or semi-transparent mirror between lens and sensor. We are talking SLR (single lens reflex) design. A mirrorless design would be better. If the design is SLR, to mount a shorter lens, we must resort to extraordinary lens design to achieve the necessary back-focus distance. This adds cost. Thus for an APS, 18mm is about the point of no return, making an economical wide-angle. Also, when the focal length approaches 18mm or shorter, we start to encounter other concerns like uniform coverage from edge to edge and distortion that will be distracting. All and all, 18mm is just about right for a reasonably priced wide-angle.
By the way, doing the math -- Mount an 18mm and the angle of view is about 68° horizontal. Most often the angle of view quoted will be 77°, this is the diagonal angle of view, like buying a TV, you are quoted the diagonal measure, makes for good advertising verbiage.