Here are a few reasons:

  - boredom
  - DIY
  - fun
  - practicing concepts, type traits, if constexprs, ...
  - spdlog has too many features, I just need one
  - why not?

Two good reasons: it is an excellent way to practice using some more advanced C++ features and existing libraries may not be able to do what you want in the manner you want. Logging libraries are also a well-bounded project you can build and refine incrementally to a high degree of sophistication as a single dev.

And of course, it eliminates a third-party code dependency in your other projects.

It's a decent question and I don't know why you're downvoted for it. I wondered the same thing.

Making it for fun or DIY is also a reasonable answer.