The alternative, having ambiguous addresses, makes systems hard to reason about and monitor, and add compplexity - eg when inevitably "internal" networks end up connected to each other in various kinds of reorganisations resulting in misconfigurations because nobody can tell anymore what the ambigous rule about a 10.xx address meant. Complexity and anbiguity are main enemies of security because you can only secure what you can understand well.

NATs are also hard to reason about in that there's no real spec about what kind of incoming traffic they allow and when. The NAT function is designed to facilitate communication in face of connectivity hurdle presented by the addressing, not limit communication.