Unless those apps not from the App Store are open source, we have very little means of being able to trust those apps to do the right thing. Apple isn’t perfect, but they do provide a modicum of assurance that the apps you install aren’t going to be malware. Apple’s incentives are aligned with the consumer — if a bunch of malware infects Apple devices, that harms the Apple brand and the consumer. The App Store also rewards developers who do the right thing by providing access to a billion devices in exchange for adhering to certain standards of security.