Form a plot perspective, finding a human skeleton would not be nearly as dramatic as a dragon skeleton. Or as obvious, since even in Orobas case, a human skeleton there could also have been a servant, dragon slayer that Orobas killed before leaving, a treasure hunter that found a way in but couldn't get out, etc (a bunch of human skeletons in the fjords would also not have been as memorable as dragon skeletons, or necessarily signified there was anything special about that location).
