Because that wouldn't be reasonable to audit. The program that compiles the new Rust compiler, as well as the programs that disassemble and decompile, could in

Because that wouldn't be reasonable to audit. The program that compiles the new Rust compiler, as well as the programs that disassemble and decompile, could insert backdoors or other nefarious behavior into the generated C, in a way that could be difficult to detect. The "ethos" (for lack of a better word) of these bootstrapping projects requires that everything be written by hand, and in a way that can be auditable.