Rust without the use of unsafe code is considered by most to be a safe language but nonetheless, it's still useful to fuzz it to find logic bugs (like assert failures) or anything that triggers a panic (like out of bound array access or integer overflow).
What planet do you live on? What do you think embedded/realtime systems, signal processing, graphics, and kernel developers are supposed to use? Also, what do you think these memory-safe, garbage collected, runtime environments are written in?
Rust doesn't yet support every platform that C supports and training staff to use a new language and adding tooling to integrate a new language into an existing codebase is extremely expensive.
Fuzzing is effective for more than finding memory related bugs. Parsing related crashed, code injection, unexpected state machine path can all also be revealed.
Beside, as much I agree with the benefits of a memory safe language, and I do believe in the urgency of promoting techs like rust, C and C++ are going to be part of our lifes for a lot of time.
If you gotta use knife, make sure you have the tools to make it sharp.
Even if everyone switched to writing Rust/Swift/$SAFE_LANGUAGE today, there are many, many projects that are written in C/C++ that aren't going to disappear overnight.
More info: https://github.com/rust-fuzz
Disclaimer: I am the author of the rust fuzzer honggfuzz-rs.