It really isn't. C is very simple.
- Complex by design
- Complex to use
C is complex to use because it is simple by design.
Though I would argue the absurd amount of undefined behavior makes it not even simple by design.
What? UB is the simplest thing you can do when you just don't want to specify behavior. Any specified behavior can't be simpler that unspecified because it's just comparing nothing with something
Rust is largely not specified, but aims to have no undefined behavior (outside of unsafe blocks).
In C undefined is used primarily when there is not a reliable and efficient mechanism for detecting a problem is happening. For example a C implementation may check every single invalid pointer deref, but more realistically it only detects only extreme out of range. So it’s undefined what happens.
That being said, at least in C++, undefined has been used largely as a joker for compiler optimizations. In Rust, if my memory serves, having the same code produce different results depending on the optimization level would be considered a pretty serious bug. In C++, it's par for the course.
I don't think I fully agree with this. A more accurate statement might be that "without a spec we don't always know which is which". You obviously don't need a spec to pin down at least some of a language's behavior, and even then the presence of a spec doesn't necessarily mean that you can answer arbitrary questions about what behavior is intended (c.f., various defect reports in the C/C++ spec asking clarifying questions, pointing out holes/deficiencies, or even admitting that certain behavior is not intended but being unsure about how to forbid it)