You can! Just not through `const fn`. Rust has macros, which at their limit are capable of running arbitrary Rust code that manipulates syntax and communicates with the compilation process, just like in a good old Lisp.
Why isn’t `const fn` like this too? One word answer: determinism.
Rust takes type/memory/access/temporal safety very seriously, and consequentially you can’t use anything in a `const fn` that isn’t fully deterministic and doesn’t depend in any way on platform-specific behavior. This includes, for example, any floating-point computation, or any random number generation, or any form of I/O, or handling file paths in an OS-specific way, or certain kinds of memory allocation. The span of things possible in `const fn`s has been expanding over time, and will in the nearish future largely overtake C++’s direct counterpart of it (`constexpr` functions) in capability. But some things will intentionally never be possible in `const fn`s, for the reasons given above.