I'm an industrial equipment programmer, so I do see C quite often. Actually, aside from C, it's just that when I first learned programming in college, I started with C. I thought that was only natural, but I guess I didn't explain enough. Inside my own head, C just feels like a given.
If you know C well already, there is nowhere to go deeper systems-wise, apart from assembly. Maybe you mean higher as in more abstract? Rust is pretty abstract compared to C. Zig not so much by design. If you want to try something else entirely paradigm-wise, maybe something like Haskell could be interesting to you, or if type systems interest you, TypeScript's is quite sophisticated.
In that case, definitely learn Rust, it will expand your thinking about programming and be useful. Zig would just be too similar to C.