That and because the vast majority of products can use a standard, off the shelf enclosure. Problem is that many of the new product designers don't come from a hardware background and either aren't aware of the huge scope of what you can buy (and get modified cheaply) off the shelf, or they think that their product must have a fully custom enclosure.

One of the benefits of having a traditional engineering education and experience is working alongside other engineers and designers who are trained in fields you weren't. It's really hard to put that type of combined experiences into a book or a tutorial.