The real reason is that in contexts like the Settings app, where toggleable items are mixed with other items, checkboxes on the left would take too much space on mobile (especially on the early small-screen iPhones), because it would require the left margin to be increased for all items. Checkboxes on the right are awkward though, and thus were replaced by the toggle switches. The immediateness argument is just a convenient excuse. Immediately effective checkboxes weren't uncommon on desktop before. In addition, there is "previous art" in the form of boolean-toggle menu items that are immediately effective and show a checkmark when active.