They exist, for instance
https://github.com/beached/daw_json_link , uses mappings of JSON <-> class members/construction. It handles a bunch of types out of the box and allows custom types to be mapped. Also, it integrates with some of the reflection like libraries out there now(Boost.Describe/Boost.PFR) and will be trivial to add C++26 static reflection when available.
Because the library doesn’t need to allocate unless the underlying types being parsed to do, it has been constexpr since C++17 too.
A bunch of people have used libraries that use macros for reflection like or PFR to integrate other C++ JSON Libraries too.