The are! It's just not an everyday occurrence - and so is worth much more - so most zero days end up in nation state hands.

Take what i say with a grain of salt, (I’m not an iPhone hacker and I follow their work semi-loosely but from my understanding there are a number of available and documented tethered exploits. I wouldn’t doubt theres a number of untethered exploits that are waiting in Apple’s bug tracker system, or are simply too abusable for the general public to know about.