There's an explanation and a gigantic list of examples here on Wikipedia.
One of the ones I've heard often, especially with newer knitters is "length of pattern equals complexity".
As a pattern writer I know this shortcut has the kernel of truth which starts the bias but unfortunately it just doesn't hold up with the changing world.
The example above falls into the category of a simple one page pattern. It's one size, hasn't got a schematic and the stitch pattern is charted. However, for a knitter who doesn't use charts there would still be a learning curve. No text instructions for a stitch pattern makes the pattern length much shorter.
Most yarn company patterns are brief as well because there are printing costs for those freebies. They are brief for some other reasons as well. Size ranges are often very limited, there is no technique support and often the pattern is in stocking stitch or garter.
The pattern world has changed with consumer demand and the availability of PDF downloads. Knitters want patterns to write out mirror image shaping, so we do. They want techniques to be explained in the pattern, so we write them out or supply a link to more detailed information. Size ranges of less than six are considered to be substandard. In my own case I have done several patterns where grading meant I wrote separate sections by size to assist the knitter in establishing a stitch pattern. Many of my patterns have both text and charted stitch patterns.
Here's another simple pattern which is five pages in length.
Look at what you get with this one. Seven sizes, written instructions, and two construction methods. The pattern is five times as long but the complexity appears to be roughly equal.