The idea here as implemented is that the unique buildings for them are quite expensive if built with hammers - it is not acting as they're common, but more like you have some basic knowledge of them, so it is possible to fund an expedition to acquire one for the special use. It is decidedly more hammer efficient to build a hunter and go catch them yourself, rather than construct it nominally.
This preserves reason for the "go hunt" minigame, both for unique and copies, while not outright requiring it.
In later years/eras, the animal buildings will be effectively cheap compared to buildings of the current time yes - but by definition that's also many years after they were probably first discovered.
Lastly herds also require a second copy, so it's not quite "just need one" (though one can, at significant hammer cost, work all your way up to tamed animals).