Because objc_msgSend exists outside of the normal ABI and calling convention.
If you were to write it in C then unless you tore out the ASM generated by the compiler manually you'd have calls to objc_msgSend in the call stack in between every method call, and even still you'd probably emit a bunch of register spills and argument handling.
The thing is, calls to objc_msgSend are _set up_ at the call site as if it were a call to the class's method's function pointer. objc_msgSend is a small trampoline that sits between the call site and the method call, it's not a proper function.
Besides, for something so small, getting a few good engineers (probably engineers who have a background in writing optimizing compilers) to hand-optimize the code and improve on it with new ideas every major release is probably the best option for everybody.
Consider also that objc_msgSend doesn't exist on its own. objc_msgSend_stret handles methods that return a struct, which is a pain in the butt most of the time, and there's also a bunch of hand-rolled variable-argument trampolines for dealing with ObjC blocks (IIRC the file is called a1a2_blocktramps.a, if you want to see something truly disturbing) that do things like calculating byte offsets and jumping to them based on how many arguments we have.
I'm going off on a tangent now but hopefully the point is made. These procedures are so far removed from normal function calls that writing them in C is not the best approach.
If you were to write it in C then unless you tore out the ASM generated by the compiler manually you'd have calls to objc_msgSend in the call stack in between every method call, and even still you'd probably emit a bunch of register spills and argument handling.
The thing is, calls to objc_msgSend are _set up_ at the call site as if it were a call to the class's method's function pointer. objc_msgSend is a small trampoline that sits between the call site and the method call, it's not a proper function.
Besides, for something so small, getting a few good engineers (probably engineers who have a background in writing optimizing compilers) to hand-optimize the code and improve on it with new ideas every major release is probably the best option for everybody.
Consider also that objc_msgSend doesn't exist on its own. objc_msgSend_stret handles methods that return a struct, which is a pain in the butt most of the time, and there's also a bunch of hand-rolled variable-argument trampolines for dealing with ObjC blocks (IIRC the file is called a1a2_blocktramps.a, if you want to see something truly disturbing) that do things like calculating byte offsets and jumping to them based on how many arguments we have.
I'm going off on a tangent now but hopefully the point is made. These procedures are so far removed from normal function calls that writing them in C is not the best approach.