> find it hard to grasp that for C++, the system's package manager (apt, dnf, brew) is the idiomatic way to handle dependencies.

It's really not about being hard to grasp. Once you need a different dependency version than the system provides, you can't easily do it. (Apart from manual copies) Even if the library has the right soname version preventing conflicts (which you can do in C, but not really C++ interfaces), you still have multiple versions of headers to deal with. You're losing features by not having a real package manager.