Because if you don't like the measure, you can go work somewhere else. Exactly the same as if you don't like how much you're being compensated today, you can go work somewhere else.

It's just that with a transparent evaluation process, you know why that is and can make more informed decisions than having your manager say "no don't worry, you're on track for a promotion next year" every year.

To put it a different way: how do you know that you aren't being compensated based on LoC output today, if you don't have some amount of transparency in evaluation criteria (and, importantly, some amount of transparency in pay, so you can verify what the company says)?