it's a legal issue at it's core.
There are too many different laws covering call recording inside the US alone let alone internationally. Imagine needing to make a different handset for every state, region, province, etc to comply. Then add in that Apple doesn't want to be a party to lawsuits and there's corporate agreements, justice dept agreements in multiple countries, etc.
What you end up with is a greatest common denominator device, or that you can't.
any app would need to interface with the phone part of the device between going in the speaker and before being encrypted. it's in Apple's hands to do.