`cargo install` is not how you add a dependency, it's not like `npm install`. `cargo install` downloads the source for and installs a runnable binary program, like `npm install -g`. Cargo does not currently have an "add this dependency to my Cargo.toml" command built-in, but `cargo add` is coming from this purpose.
With `cargo install`, the default behavior is to completely recompute the dependency tree before doing the build. The `--locked` flag modifies that to use the lockfile included in the package to do that build instead (and in fact fail compilation if it does not exist). That lockfile will still be a full graph of all dependencies and transitive dependencies to build the binary, it doesn't like, recurse or use any lockfiles that exist in any of the dependencies' packages.