Raw C++ is unsuitable as a build language, so you'll have to layer a DSL on top of it. Either way you're learning a new language.

As to "why learn all the quirks of CMake," the selling points of CMake are its widespread availability, maturity, and coverage. For example, just on the packaging side, it knows how to create tarballs, .debs, .rpms, Windows .msi files, multiple flavors of Mac installers...

Less mature build tools require you to implement this stuff manually. CMake mostly handles it for you (and has the scars to prove it).