Go's goroutines aren't plain C threads (blocking syscalls are magically made async), and Go's stack isn't a normal C stack (it's tiny and grown dynamically).

A C function won't know how to behave in Go's runtime environment, so to call a C function Go needs make itself look more like a C program, call the C function, and then restore its magic state.

Other languages like C++, Rust, and Swift are similar enough to C that they can just call C functions directly. CPython is a C program, so it can too. Golang was brave enough to do fundamental things its own way, which isn't quite C-compatible.