It's not really a basic feature. The sandboxing of iOS apps is smart in many ways. Allowing 3rd party access to an native Apple app could open up problems.
But there's a good reason for that -- as long as legislatures keep making it ILLEGAL to use such a feature, can you really blame Apple for not wanting to offer it?
In some states ("multi-party consent" states), it is illegal to use the feature without the consent of the person being recorded; just like in some situations it is illegal to use the "camera" feature without the consent of the person being recorded. That doesn't stop Apple for offering a camera on their phone.
Other states have "one-party consent" laws that mean it is always legal for you to record own phone calls.