I have not heard this 'criticism' ever before and it's absolutely ridiculous on so many levels to me as a person tangentially interested in gamedev, C, C++, runtimes, system programming, retro stuff, history of gaemdev, etc.
1. Why would a web dev suddenly decide to write in such a spartan way an algorithm heavy library in C or C++? This makes no sense except to somehow tie modern web dev failures like left pad with this style of libraries to portray them in negative light...
2. In gamedev neat organization is/was not some golden long standing standard, i.e. in comments under[0] you can find information that with 3D graphics it was customary to use a single file to let the early compilers do a better job since they didn't optimize across translation units and many old codebases have no problem with thousands of lines of spaghetti per file.
3. These libraries are meant to be compiled once ever per project (except when you clean out all your object files) with a define in a single source file (which any modern compiler should handle), otherwise most of their text is thrown away in a single ifdef which should be trivial and speedy on a modern compiler. The resulting header shouldn't be a problem for any modern compiler and is as light or lighter than an entire include tree would be. Most of these libraries are also single purpose and have very small API and C is light compared to template heavy C++ anyway. If you develop such a library you can keep it split up and ship merged files, i.e. SQLite does that. If anything this might speed up the execution (optimization in single TU), compilation (a single TU for entire library) and linking (no dynamic nor static linking but a single object) but to me that tangential compared to convenience itself.
4. I've never seen such libraries to have been badly received - quite the contrary, SQLite might be the most widely deployed piece of software actually and it's recommended consumable form is a single pair of files (plus an ext file, for a total or 3 in the official 'amalgamation').
5. I've personally yet to see a 'young web dev' create such a library. If anything it's the grizzled veterans of gamedev, runtimes, compression, middelware, cross platformness, systems programming, people with university education that includes CS heavy stuff, etc.
Libraries I use or like that use this (single file or single header + source) style are:
- very popular PD libraries by Sean T Barret, thanked in this linked repo's README. He arguably popularized this style, maintains a list of such libraries and has a FAQ and a guide about this style of libraries. He is over 50 (or maybe over 60, he was a teen in the 80s), is a gamedev veteran with software and hardware graphics and runtimes - Thief 1, Iggy (Flash runtime) at RAD, etc. His website is also hardly something that a modern web dev would make[1].
- pugixml by Arseny Kapoulkine - gamedev veteran: PS3, physics, rendering, etc. not a web dev, not young (definitely over 30, unless you happen to redefine young to 'under 40' which makes 0 sense in industry as young as ours but considering your other claims is totally likely).
- SQLite which is shipped and recommended to be used like that (and I think Python embedds it like that) - D. Richard Hipp, Tcl, over 50 now, total veteran of world class between Fossil, Tcl, SQLite, etc., educated and programming since 90s, his personal website is also hardly modern web dev.
- ClipperLib by Angus Johnson, a hardcore clipping 2D library, it has single/two file versions in C#, Delphi and C++ (are these the languages young web devs know well now too?) - another person who was a programme rin the 90s and is from very Enterprise-y background (Delphi Object Pascal, yikes), not a web dev, nor with a web dev worthy web page[3] (I don't mind it, I even find it nice to look at, but it's another hint of the lack of supposed web dev blood in him).
Honorable mentions are the two people related to imgui concepts and also thanked in this linked repo's README:
- Casey Muratori - programmer in the 90s already, over 30 or 40 years old, another (along STB) RAD tools related person, worked on Granny (animation for games) and Bink (video compression for games), total veteran in low level stuff, not a young web dev.
- Omar Cornut - maker of Dear IMGUI, was an intern in the 90s, has a very non web dev website[4].
All in all I'm truly baffled about where you got the 'mostly young people who come from web dev' bit, especially since it's no secret many people are inspired to write them by STB himself (and mention that in their READMEs, as seen here) who is none of that. Your comment seems very hearsay, dishonest disingenuous.
If anything I feel like this is a set up of a Yerevan (communist, post communist, Eastern Europe, Eastern Bloc, [5], etc.) joke I happen to know in Polish (translation mine): "Dear Radio Yerevan, is it true they give out free cars on the Red Square in Moscow?" - "That's mostly true but it's bikes, not cars, they are stolen, not given out, and not in the Red Square in Moscow but in the Prague district of Warsaw".
[0] - http://fabiensanglard.net/duke3d/
[1] - http://nothings.org
[2] - http://www.hwaci.com/drh/
[3] - http://www.angusj.com/delphi/clipper.php
[4] - http://www.miracleworld.net/
[5] - https://en.wikipedia.org/wiki/Radio_Yerevan_jokes