Because of the indirection and complexity it causes.

The user is in a shell. The simplest way to add something to their system is a shellscript. No moving parts. You get what you see in the script.

The way the project is set up, there are multiple moving parts. The interaction between the shell, some compiler and a bunch of rust files.

Imagine during the moon landing, Neil Armstrong could not have communicated directly with Buzz Aldrin. But they would had to go through a translator who translates English to German. Who passes the message on to a translator who translates it from German to French. Who then passes it on to Buzz. BOOM!

                      +   /\
         +              .'  '.   *
                *      /======\      +
                      ;:.  _   ;
                      |:. (_)  |
                      |:.  _   |
            +         |:. (_)  |          *
                      ;:.      ;
                    .' \:.    / `.
                   / .-'':._.'`-. \
                   |/    /||\    \|

By that reasoning all command line tools must be shell scripts.

I don't think this tool changes the shell command for you? Your analogy doesn't fit here.

But I do agree that it is probably easier to implement as a shell script.

shellfirm not changes the shell command, only get the the command and check the pattern. the shell is execute the command.

You really think shell script will be easier? can you elaborate?