If you look at other languages, automatically-generated code documentation has become the main way people ingest documentation for software projects. Javadoc was the first major such tool; if I'm trying to use a Java library, I don't read the source code, I start by reading the generated documentation. If you've ever used https://docs.oracle.com/javase/8/docs/api/, you've used the results of Javadoc. Similarly, Rust also has a widely-used builtin documentation tool (and again, the official standard library documentation is entirely the result of rustdoc).

C/C++ of course don't have a builtin tool. Doxygen attempts to fulfill that same gap, but it also suffers from trying to support every language under the sun, as well as the inherent issues of C/C++ (the use of the preprocessor to generate module boundaries does not make for fun processing). I also find that the default settings tend to make too many graphs that aren't really useful but take up a lot of visual space (I've never found collaboration graphs to be helpful), and its approach to trying to figure out what things are types gets pretty sketchy (why does it think that unsigned is a class?). To top it off, I was never impressed with its design aesthetic.

In short, the fault of doxygen is that it's not designed in close integration with the language parser to ensure a highly correct understanding of source code, which makes it tend to fall over if anything is moderately unusual or confusing. And since the general experience with its output tends to be negative, there isn't much effort in trying to maintain quality documentation comments in the underlying code in the first place.