I'm a bit confused about what exactly the definition of a "perfect enemy" is. Is it just based off their combat prowess and how difficult they are? Or is it their backstory and lore that makes them a good adversary?
Neither one, it's because the Enclave troopers drop all the 'ph4t l3wtz' that an apparently sizable portion of the fanbase lusts over.
At least, that's the impression I got from the incoherent rants posted by a couple of people in this thread, I could be mistaken. It certainly couldn't be due to any challenge the Enclave troops present, because there isn't any.
As far as Enclave presence in the Chicago area is concerned: if there
are still a sizable number of Enclave personnel there, the
last thing they want to do is make themselves known as they are surely aware of what happened to the groups on the coasts. Besides which, they probably long since realized they have no chance at reestablishing the organization, and as such probably just want to be left in peace in their old age.