Even for database engines specifically, modern C++ is essentially as safe as Rust and significantly more ergonomic. Rust's safety features can't reason about the case when all of your runtime objects live in explicitly paged memory with indefinite lifetimes and no fixed memory address, which is the norm in database kernels. You have to write the same code to make handling these objects safe and correct in Rust that you have to write in C++. You can't use normal pointers and allocators for this even if you wanted to.

Rust's safety is designed more for normal dynamic memory applications.