To play devil's advocate, do you think in an alternative universe where Apple products have built-in easy to use native support for jailbreaking with the push of a button that some large malware outbreak that only affects those devices jailbroken devices won't be blamed on them anyway?

Apple has other motivations to maintain their walled garden, but their goals of controlling the whole experience also extend to things like that. They want there to be no question that any Apple device is "safe", regardless of legitimate user configuration.

And as long as users at large aren't demanding general purpose computers who can really blame them?