LOL. That security bogeyman argument is a convenient post-hoc justification. The real reason is much simpler: Because "computers" started out with "Owner can run code of their choice on it" as one of the main definitional qualities agreed upon by the market.

Devices which only ran code that the vendor had specially packaged for the device always had other names, like Games Console, Word Processor, MP3 Player, Graphing Calculator, or just plain "Toy." Nobody confused those with computers and nobody accepted them as computers.

Phones entered this world already in that second category, long before Apple even considered making one. Apple kept an iron grip at first and after careful consideration, decided to only allow third-parties to even enter on terms extremely favorable and profitable to Apple, and always under their direct control.

Closing off the Mac would be very messy and also cause tremendous headaches due to it being the development platform for iOS. They don't want to bother. But the reason iOS is closed is because it's always been closed, and they'll never open it unless forced, because money.