For the same reason you’re running AdGuard and not just pointing all your devices at the recursive upstream.
You’re using AdGuard / Pihole as an ad sinkhole, not just to cache and forward DNS requests. Like if you really wanted to you could hack together something in Unbound to do that, but why would you do that when Pihole already exists? You have two things built for purpose.
I mean if you want to build something around Unbound to do ad blocking and set up a monitoring stack for metrics and all that jazz that’s great, more power to you. But you already have two things built for purpose, there’s no reason to go out of your way to do that. And I don’t think OP here is prepared to do all that.