Homebrew tries to install binaries by default. (They call them bottles) Building from source happens if a suitable 'bottle' isn't available, or when `--build-from-source` is specified with the install command.

I know cargo is installed only once, but I don't want cargo. I don't build Rust software myself, so I don't want to have it hanging out on my system taking up space purely just so I can have one or two useful programs that were written in Rust and depend on it. I'll just go with some other alternative.