Actually many people have a preference for writing their Python libraries in C :-) Python was explicitly designed under the assumption that much of the heavy lifting will happen in C. And C programmers write their parsers in Yacc, etc.

(This is not to say that rewriting libc is a good way for people knowing Rust to spend their time. It's a ton of effort with the result most likely to be unused and if it is used, then it provides a small benefit to C and C++ programs at the expense of a rather scarce resource - Rust developers - having been diverted from benefiting Rust programs. If I were a Rust enthusiast, I'd much rather write libraries in Rust designed to be used in Rust programs so that more such programs get written more quickly than use Rust to put lipstick on the pigs that are C and C++.)