Because, if you program in a type-safe language, if you call something that is compiled separately from you, you'd still like to maintain type safety across that call boundary.
You absolutely get type safety across module boundaries with C in that if provider and user both compile against the same interface, this will be typesafe.
You could even have this type safety on the linker level as far as C is concerned. You just need an object file format that exports C types for symbols. This is not done on any of the (few) systems I know, and probably for practical reasons.
Some other languages give you this link time safety, but I assume at the cost of less interoperable object files.
