Why is what, why is it default or why is a resolver better than a forwarder?
As to why its default, ask the developers. But a resolver with dnssec enabled is better from a security point of view because as long as the domain your asking for a record from is using dnssec you are assured that the answer you get back is legit and what the authoritative server wants to hand out. Not some record in a cache, its quite possible the stuff in a cache has been poisoned, etc. Or maybe your cache entry ttl has not expired, or maybe the ones running the cache don't adhere to the authoritative servers ttls settings? To reduce their traffic?
If your asking the authoritative server you getting the answer right from the horses mouth, not some cache. To me this would always be the preferred method. Be it they sign it with dnssec or not. But in a perfect world all dns would be using dnssec..
Look at it like this, if the developers of pfsense thought it was better or a preferred setup to use unbound in forwarder mode - why and the F is out of the box in resolver mode ;) That would make zero sense. So its logical that pfsense using unbound in resolver mode is the preferred method..
You could debate that since some users have problem with resolver mode because they on some bad isp that blocks dns to anything other then their servers, or they intercept dns traffic and do something odd with it. Or maybe the user is just on a bad connection where talking to the authoritative servers for a domain on the other side of the planet takes too long. So in that case the forwarder to the users local isp dns would answer queries better. That really should be one offs to typical connection, and those users should make the decision to either bring up the issue to their isp, or get a better connection or use the forwarder.
I think its better to deal with the few users that have issues, and put most of the users than don't really know any better on a better security footing then leaving the default at forward mode and hoping the users read and figure out on their own that resolver is better and more secure.