Do some technical documentation websites purposely have very plain design?

I notice some widely read/important websites, all technical documentation pages in my examples, have some really old, dull and seemingly poor design.

Is this on purpose? Is it based on UX considerations?

I imagine the maintainers of these websites have the resources to build a nicer looking website or even have the skills to do so themselves, so rather than assuming they're just being lazy, I wonder if there's an actual reason why the design is so simple? And, if there is, then I'd guess it for UX reasons (even though I have no idea why. Very little navigation/searching ability, no colour coding, etc).

Any guesses?

Here are some examples I'm talking about:

GCC - https://gcc.gnu.org/install/configure.html

A random page from a binutils command docs - https://sourceware.org/binutils/docs/ld/Options.html

A random page from Linux manpages - http://man7.org/linux/man-pages/man3/acl_calc_mask.3.html