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.
I've probably been reading too much lately about FOSS developers not being able to find help maintaining packages. But yeah, it's not a helpful comment.