It all depends on the design of the lens and what is most important to the designers. Ease of use regarding size and weight? Low cost? Optical performance? Weather sealing and durability? Wider/constant maximum aperture? A wide range of focal lengths?

The reason not all lenses are designed with internal zoom, constant aperture, and internal focus is that different lenses are designed to do different specific tasks and to be produced at specific price points. In the case of your kit lens the consideration of keeping the cost lower and allowing for a wide range in focal lengths necessitate the use of an extending lens barrel.

Most internal zoom lenses have a smaller ratio between the widest and longest focal lengths they can do. Common lenses with internal zoom are 70-200mm in which the longest focal length is just under 3X the shortest focal length. There are very few lenses with internal zoom that exceed that 3X ratio. Lenses such as 24-105, 70-300, etc. almost always have a barrel that extends as one zooms, even the more expensive ones.

The same is generally true with constant aperture zoom lenses. Most constant aperture lenses also have a 3X or less ratio between the longest and widest focal lengths.

If one were to try to extend the designs of, say, a 70-200mm internal zoom/internal focusing lens to be a 70-300mm lens the internal zoom elements would bump into other elements of the lens before they could travel far enough to extend the focal length to 300mm without extending the length of the barrel.

For an extensive comparison of the various designs of lenses including examples from the Canon lineup, please see this answer to: How does (auto-/manual-)focus changing work?