Because it's not the 1980s anymore, and the thinking around security is different today. Running arbitrary code was mostly fine on 8- and 16-bit micros not connected to a global network, but today we need some sort of attestation that the code we run is trustworthy, otherwise it must be assumed not trustworthy. Code signing plus whitelisting -- the App Store model -- is one of the easiest ways to provide some degree of assurance of a program's trustworthiness.