I would say simply for me it is lack of knowledge. Not that I don't have the ability but there is a lack of resources online for learning how to write pure JS/CSS/HTML websites. There are doesn't seem to me to be any good resources or guides to file structure or ways in which to properly go about things.
Frameworks make it easy to get started quickly. But I would like to get into more into it I just don't have the time myself. I don't have the time to be a purist. I would believe that many developers might be working like I do and simply build what is requested at work. Be it a phone application, a desktop app, a bit of ML software, or a web application. I don't have time to be pure in all aspects and simply need the job to be done and have myself maintain them.
However I still would like to learn more about a pure JS/HTML/CSS website and design. Maybe someone should make a large course on Udemy for it.