That's perhaps because all these kernels were written before C++ exists? The first C++ standard is from 1998, and stable STL/C++ support is even more recent.
FYI : in NT many device drivers are written in C++
FYI : in NT many device drivers are written in C++
Not one production-level kernel that is in wide use as a general-purpose operating system uses C++. I don't believe that to be a coincidence.
(Nitpicker's corner: Darwin's device tree subsystem is written in Embedded C++, an extremely cut-down version of C++ that's more like "C with classes" than C++).