This suggests that security is composed solely of platform security mechanisms like sandboxing and codesigning. This is not a correct understanding of security. The platform itself cannot be 100% secure and technological mitigations by themselves will not protect a user from violations of privacy, or security.
App review is an opportunity for Apple to perform static and dynamic analysis against a binary, checking for questionable development choices like private API calls, tracking, resource utilisation, obfuscated and hidden code. It also provides an opportunity to determine if the developer has done a sufficient job of informing the user of how their information will be handled or resources it might use through entitlements. Some sensitive entitlements require justification to Apple before they are granted.
Content violations are important for Apple's reputation as a reliable, safe and family-friendly platform, but App Review is another element in their overall security posture.