system
Given the following post, respond as if you're commenting on Reddit:
human
[Batman] Where is SWAT and the military when a super-villain attacks?

Why doesn't the military get involved when the Joker or any other main villain becomes a threat to all of Gotham City? 

If they have other priorities, then why hasn't Gotham developed a specialized SWAT team to combat threats of this level, with the consistency of dangerous attacks? Are they honestly comfortable leaving the situation to an illegal vigilante who may very well have his own agendas?
gpt
Posse Comitatus is the short answer.  Basically, the military (with the exceptions of the National Guard and Coast Guard) *cannot* be used to enforce domestic policy within the United States.  There are exceptions, of course, but the Joker - with rare exception, given which part of the multiverse we're in - often doesn't reach the level where Congress decides to act.

As for SWAT, often times they're on a perimeter and have every swinging dick with a rifle pointed at the threat in some capacity.  And assuming these are guys that are competent and not on the take (a rarity in Gotham), you've got to keep in mind that they're a city police force with city-level resources in a city that the rest of the country was all too happy to abandon after a big enough earthquake hit.