Jeans are a type of pants and are usually made from denim or overalls. Jeans can also be made entirely of cotton, similar to denim. High quality jeans are made of a stronger and heavier denim fabric, which sets them apart from cheap jeans. It is true that high quality jeans can look a bit heavier or stiffer.
Jeans have been around for centuries. They were made like work clothes and had a practical purpose. This is why it is made of denim. Durable, versatile and economical fabric. That's why the pockets are designed to be easy to access.