In my experience, most development comes from two motivations: employment or recreation. There are others such as creating open source software (like hapi), research, and the like, but most of them fall under the aforementioned two categories. The motivation behind both categories is usually result-focused; when the motivation is employment, we aim to generate a monetary value from the code that we write. In case it is recreational, we are often exploring a new technology and just want to build something for a demo or to learn the inner workings of a new framework or library. It's easy to see how writing tests doesn't fit into either category.
In case of development for employment, tests aren't a sellable feature of software and are expensive. They take a lot of time and money to design, write, maintain, run, and keep up-to-date with best practices. Often, the size of a testing codebase will be larger than that of the codebase it is testing. The...