Because most customer service agents don’t understand American Sign Language, and it’s quite hard view sign language over a phone call, hence the whole sign language interpreter thing.
Because (a) video chat doesn't work brilliantly with sign language (signing can be rather quick, so any latency or artefacts could results in words or expressions being missed, and (b) not every Apple CSA knows how to sign ASL -- most don't.
Most places don't have ASL interpreters on-call or even to any degree among their staff, and often rely on external companies to provide (often pre-booked) ASL interpreter sessions to D/deaf clients.
It's really nice that Apple is making this available so easily and accessibly, and in-house. Just being able to click and connect to an Interpreter is a great improvement in accessibility for ASL users.
