The choice is most likely deliberate: this is a 9front crowd (Plan9's aesthetic choices are rather peculiar). It's what man(1) should display on a 80-column or so terminal.
Debatable: 80 columns ease reading, as each line is completely within the cone of vision. It's not enforced in their style(5)[0], but the code mostly and naturally (e.g. short variable names) falls within this limit (random sample[1]).
(I don't think that Plan9's "not thinking in terms of terminals", which is more of a "a better-than-X11 GUI is the default UI" is really relevant in this context)
EDIT: I don't have a Plan9 box around, but at least on plan9port, man(1) by default wraps lines to 72 columns or so.
Groff's HTML output is very poor. It's a sad day when it's more convenient to generate postscript and convert to PDF than it is to read the directly generated HTML output...
I think it may have something to do with display:flex. CSS isn't a strength of mine but looking at the computed view, there's a wide inflexible margin on the right side that causes the text to wrap before it otherwise would need to on smaller screen widths. The nav causes the same on the left side.
[1] https://en.m.wikipedia.org/wiki/Man_page