Because "the government" is an abstraction. In reality the decisions have to be made by specific people with specific interests and biases.

People with regulatory decision-making power are, by definition, powerful. So they have the interests of powerful people, which are unlikely to match the interests of most of the rest of us.

More importantly, government is an organization.

The foremost motive for an organization (like any organiam) is self-preservation. Transparency is usually perceived as a vulnerability that might allow enemies to attack.

And when enemies == constituents, it might be time to fold the whole thing up and start over.