Because everything is a tradeoff. Getting the binary to be smaller means taking more compile time, because compilers have to do work to reduce the size. It is much faster to produce larger binaries.

Additionally, most compilers produce something that's useful for development and debugging by default, and that means including extra stuff for that purpose.

A big part of why binaries are "big" is dynamic linking and external symbols.

That Rust hello world is just hand-crafted to never be linkable to anything else at runtime and invoke a system call with a buffer. This isn't really how we'd like to do most things.

Just like C compilers, it is a matter of use case, and compiling for size has tradeoffs regarding execution speed.