edit: aside from the new list of "classless CSS frameworks/themes" mentioned in @networked's post.
You only have to set the margins/width of your document and maybe some sort of header & footer and you're done with the CSS your very basic but also very readable website.
I am not in a position to doubt your stated preference, but certainly "it looks rather good to me" does not block you from understanding why others would would want some basic styles applied to the sites they create and visit.
I don't see any linebreaks in the source, but there is this:
input:not([type='checkbox']), select {
display: block;
}
Alternatively: the defaults are poor.
Fonts which are too small or large, or text lines which are too long, are objectively harder to read. Other design choices similarly have measurable impacts.
Jakob Nielsen's methodology of timing task completion and measuring completion accuracy is a UI/UX testing methodology which removes subjectivity.
I compulsively CTRL+wheel-up and CTRL+wheel-down all the websites that I read until the "right" size appears, so I do not really care about the font size setup by the author of the site. But probably my behavior is not very representative.
A bit from this discussion https://news.ycombinator.com/item?id=19607169