The authors are required to make it obtuse. They're required to use warts on all of the names because most of the code is in the head files and is generative code compiled by users of the library rather than the vendor. In order to avoid naming conflicts they can only use obscured names in their implementation of any but the defined API (eg. naming any internal functions, macros, or variables with leading underscores).
So, the authors did intentionally make it as obtuse as possible for your benefit. It's written to be used, not studied, by all kinds of developers in all kinds of circumstances.
They could supply a "pretty" version for people who want to review it. Every time I have to step through code (and accidentally step into STL code) it looks sloppy and gross, like a swamp. No comments or organization. I would expect something neatly formatted, and comments saying "This is overload-4 of std::copy()..." etc.
Professional software developers have a lot to do just to get their job done on time and within budget. Having to duplicate all their code just so that people who contribute nothing to the end product can have an easy time understanding it is just never going to be a priority worth addressing.
The problem here is not really the code, it's the reader.
Who says they have to duplicate it? Just write the original version clean, clearly, and concisely. Then run it through a mangler to rename variables to avoid collisions.
The STL is maintained by volunteers, it's a FOSS project. So your appeal to Serious Business doesn't hold.
So, the authors did intentionally make it as obtuse as possible for your benefit. It's written to be used, not studied, by all kinds of developers in all kinds of circumstances.