I'd have to disagree -- the lack of OS-level sandboxing primitives such as seccomp-bpf and SELinux[1] means that exploits happen rather regularly in iOS rather often ([2], among others).
iOS has a perfectly good sandboxing model that is literally called "the sandbox". You will note that the impact of that bug is limited to the process it is triggered in for precisely this reason.
[1] https://www.ise.io/wp-content/uploads/2017/07/apple-sandbox....
[2] http://www.trustedbsd.org/mac.html
[3] http://www.trustedbsd.org/sedarwin.html