I’m pretty sure the default max-width for a platform such as this has to be based on best practices and those best practices are based on research. From my ancient past when I did web dev, and even more ancient past doing desktop publishing, I remember reading about how the human eye will be exhausted more quickly if line lengths are too long. I think it was around 40 words or so. Anyhow, on the web, most style guides would propose such a max width, allowing for easy reading and enough whitespace around the text which is also easy on the human eye when reading large bodies of text. From a purely aesthetic perspective too it makes a difference to most people, a block of paragraph text with margins is more appealing than single long lines that span the width of the screen.