Security is a trade-off, not a slider to maximize. A machine with no network connection and no ability to install new software would be ultimately secure, but not much use. Caring about your users means optimizing the ratio of security to usability.

Yet you can dowload apps outside the app store and set as low security as youd want.

The app store has nothing to do with that as most normal users dont download anything.

The mac was considered secure long before the app store.