You found a promising library. To actually try it, you clone the repository, install a toolchain it expects, wire it into your build system, and resolve its dependencies. One hour later, before you have written a single line of your own, you are debugging someone else's CMake. Most people never get that far and simply close the tab.
You found the perfect C++ library. The code is clean, the benchmarks hold up, and the
license fits. Then you go looking for the documentation, and the only thing you find is a
README, a little longer than most.
A stranger found your library. They have not read a line of your code yet. They glance at the CI badge and the date of the last commit, and in about thirty seconds they decide whether the project is alive and safe to depend on. A red badge, or a master branch last touched two years ago, and they leave. You won't hear from them about it.
You need a units library for C++. You search GitHub for "units" and get a wall of repositories, all some variation of the same word, by different authors, with no obvious way to tell which one is alive, which one is serious, and which one will still be maintained next year. You pick one almost at random, or you give up and write your own. The library you needed might have been right there. You just could not find it, or could not tell it apart from the noise.
I can tell you exactly how many stars mp-units has. The number is right there on the GitHub page and on my docs site, it updates in real time, and I can plot it against every competing units library on a single chart. What I can't tell you, even roughly, is how many people actually use it.