I highly doubt that's a reason, seeing that the watch can maintain just a phone call for 3 hours which is much more intensive, requiring a persistent connection, than Siri speaking back could ever be.
I think it's just about discretion and I appreciate it.
I used to use "Raise to speak" with Siri all the time, speaking as if I was naturally on the phone, but then they broke her so you HAD to pull the phone away from your ear to finish the request and she would speak out loud, which ruined Siri usage for me. I don't even know if they've changed the behavior since, it was just so off putting.