There's multiple reasons companies don't want to publish salaries, but they almost all boil down to wanting to pay people as little as they can get away with

Good companies see employees as partners, make sure incentives are aligned, and use transparency to empower their employees to make better choices with full awareness of the entire business. It's amazing how much more productive an employee can be if they believe in the missions, understand exactly how their work can benefit themselves, and have genuine trust that their employer has their back.