...this is getting weird."Need" how? The manga seemed to get by fine with depicting her as a human-form robot. Why couldn't the live-action version do the same?
The people who were making the movie made a decision to do Alita as CGI. It worked, giving her a look that was simultaneously alien and like a young teen, which is what she needed to be.
But even aside from that, "I demand that you justify why a particular decision was made in directing a movie" is a really odd approach to take, at least unless that decision was catastrophic or evil.