Because they were designed to be read on a terminal
man pages are designed to be shown in a terminal. As such, there is a very basic set of formatting available. You can read about the format on roff(7), but basically your target was a terminal or a printer. There is some formatting available, but you can't count on color-coding something that I will read over a telnet link (ain't I lucky?) on my monochrome phosphorus screen.
You can read about the headers and conventions used man pages at man-pages(7). This provide coherence even though they are typically developed by completely different people.
info(5) format provides somewhat more options, and you will find long manuals in that format, as opposed of man pages that fit in about a single page. gcc and binutils documentation are actually written in texinfo, from which it is then compiled into different formats: man, info, html, pdf, etc.
This is actually since the developers can write it once, and have the same content on multiple formats. It is better not needing to keep the versions of several documents synced (some projects do, though). The GNU project states that the documentation MUST be available in info format, which leads to its prevalence.
You could have different themes applied, but you are restricted by the actual structure of the underlying actual content. Some projects choose not to make man/info pages available, such as 7-zip whose docs are in html. I find that annoying, but that may be just because I am used to finding the manual of utilities directly from the terminal. Nowadays, it is common to be connected to the internet, but it didn't use to be so. Plus, accessing the manual page from the actual system -to which I may be connected through ssh, so no fancy images- means you get the manual of the version you have currently installed, not a future version or a different flavor. Compare sed vs sed vs sed vs sed vs sed.
Finally, you should take into account that technical documentation is usually made by developers. Which means the technical documentation is, well, technical, with the goal of communicating a few select points, like the parameters that it supports. It is good to be consistent and familiar to the developers. So bells and whistles wouldn't be appropriate.
Do note that info manuals do show (in HTML) a navigation structure, however. They may not feature a search engine (that feature would require server side support, whereas generated files can be just uncompressed in a web path), but they do offer a one-page version, where you can easily Ctrl-F from your User-Agent any desired term.