The lens acts like a funnel in that it gathers light. The larger it’s diameter, the more the surface area, the more light it gathers. As light rays transverse the lens, the density of the material and the shape (figure) of the glass alters its direction of travel. The shape mimics a lentil seed hence the name “lens”.
The camera lens is a converging lens. Light rays are caused to turn inward. We can trace their revised path. The trace reveals that the light rays take on the shape of a cone of light. This action is called refraction from Latin to bend backwards.
Now a short focal length lens must have a steep figure. We are talking, thin edge, and thick in the center, This shape is like a slice off of a tiny glass marble. A long focal length lens has a gentle figure, while still thin at the edges; these are less fat at the center. What I want you to know, a short focal length lens is a stronger magnifier while a long focal length lens is a weak magnifier. It is by far easier to make a long lens and super difficult to make a short lens.
We can make a lens with most any focal length. Short lenses give a wide-angle view whereas long lenses deliver a telephoto view. Everything is OK when making a fixed focal length lens. Nowadays these are called “prime” lenses. This is opposed to a variable focal length lens, nowadays called a zoom. Think of the complexity of making a variable focus lens. You add a bunch of individual lens elements and stack them in a tube. Now you must construct a mechanism in the tube that moves the individual lens elements, changing their spacing to each other and to the camera body.
As they move, errors creep into the design. These are called aberrations. The lens maker must deal with seven major aberrations that distort and contort the resulting image. Not easy to do when making a “prime” super difficult when making a zoom. In a zoom, the nature of the aberrations change as the focal length change. To counter more lens elements are needed, each with a different figure. Some are cemented together, some are air spaced. Some are dense glass, some are light weight glass. Each added element has two polished surfaces that reflect light. This reflected light is lost as to making an image. Worst, the reflected light mingles with the focused rays and interferes and baths the image with flare.
It’s not impossible to make a broad range zoom but it is not easy and good ones must also be affordable or the design just sits on the shelf. You only make money if the lens sells. It only sells if its good and priced so people can afford to buy it.