It seems clear to me that the calamities are obstacles which have to be avoided at all costs.
If they would be easy to nuke like meruem, then the humans would not be so afraid of them.
The 5000 ants under paristons control are actually far more dangerous than meruem, because there is no way to get rid of them easily.
No matter how strong meruem is, with enough firepower, you can always abuse some nen ability to get close to him and then bomb him.
If all else fails, you nuke a whole country, his en was incredibly far and he can instantly teleport, but even that would not help him against a tomb of well timed nukes.
But having well trained ants run arround and possibly found their own colonies in rural areas, that is a devastating problem. You can't just nuke the whole earth and they outnumber the hunters greatly and you'd possibly not even find them before they reproduce.
And yet, ants are only rank B in danger level.
This cannot be due to pure strength alone. Meruem seems to be the epitome of power, but he fell to poison.
This might be an interesting analogy: The allpowerful meruem fell to an even bigger threat: Humans. And they are no bigger threat because they are stronger than him, but because they have means at their disposal which are simply hax.
I believe the same is true for calamities: You cannot just wipe out one very powerful creature (or at least in many cases) with a nuke or due to a good combination of nen abilities.
You cannot simply wipe out a disease like zobae.
And i think the rest have means like that at thweir disposal as well. Maybe you can destroy them, but they simply regrow and keep spreading (like a plant, which brion is).
Maybe gas entities like Ai do not even have a tangible body to destroy.
For some time, i imagined the calamities like the 'gods' of that realm, but i think otherwise now.
I think the calamities might not even big a big problem for the other wildlife or civillians on the DC, but specifically for the humans.
Lots of other creatures might be immune to the danger the calamities produce.
Humans, in this picture, are like fireants. They can wipe out about any other tribe of insects with their superior technology and nen (even the ants are not a big problem if they don't eat a human who has nen), but there are things they cannot fight back against, like parasites, some flesheating plants which attract them.
Or even some fungus which makes them lazy zombies and stops their reproduction.
The ant simply has nothing at their disposal against that, because all its strategies fail.
Once the infection spreads or the ants are on a wrong territory, they will die or their tribe falls apart.
And i imagine the DC to be a place like that for humans, where they have to proof they can take it head on with careful planning and observation.