I'd also argue that it _is_ surprising, as we'd expect the majority of unknown species to be similar to known species. It's not like every unknown species is just randomly created-- most will have ancestors that are well known and studied.
Considering that leaf-cutter ants are well-known to humans (I've seen them in a museum in Amsterdam), I'd say it is surprising that we haven't noticed this before.
I spent this summer learning about the different mushroom in my area in as much depth as possible and it is incredible how little we know about most of them besides a name and very apparent superficial features.