A lot of us are too busy solving problems. Learning about the latest language features, which we often won't be able to use anyway due to the trouble of moving a large dev environment to a newer standard, feels like academic masturbation.
C++ folks are very much into their language, and can't seem to understand that most folks don't want to dedicate significant amounts of mental resources purely to language details.
Moving to new C++ is a non event, change the compiler / flags and done. Using the new features requires some learning but not a big deal since you can figure out what you need from a summary and learn what is useful for your problem.
The problems of the code I'm writing far exceeds the complexity of the language. Your complaint about complexity fall flat to me, unless you are working on a trivial program you need to deal with things far more complex than any language.
GCC was implemented in C and there are plenty of other C compilers written in C. GCC has been converted to C++ at some point, but large parts are still essentially C and I do not think the change to C++ was actually helpful (but others may disagree). In any case, the idea that one needs C++ to have C compilers is certainly simply wrong.
Clang/LLVM with its more permissive licence sees _much_ wider use and is written purely in C++. PlayStation, automotive, platforms and runtimes like Chromium/V8, Android, etc. are all built with Clang.
Naturally GCC was originally written in C, given its age, and the original GNU coding standards document.
With time, the GCC developers acknowledged the benefits of using C++ over C, and migrated the code.
GCC requires a C++ compiler to bootstrap since around 2012, and GNU coding standards has been updated to several languages beyond C, time to go up with times.
C++ folks are very much into their language, and can't seem to understand that most folks don't want to dedicate significant amounts of mental resources purely to language details.