I wish they had episodes with Villians teaming up!

Technically, they did.

Poison Ivy and Harley unintentionally teamed up, but stuck with it for an episode.

The Penguin, Two-Face, and Joker teamed up -- altough on a financial basis -- in "The Strange Secret of Bruce Wayne".

And in the Batman/Superman crossover two-parter, "World's Finest", Joker teamed up with Lex Luthor.

And a lot of them kind of came together to get Batman in "Trial".



But as it was, team ups weren't really needed -- the villains worked so well on their own.

_____
The New Number 2: "Are you going to run?"
Number 6: "Like blazes. First chance I get."

There are a few reasons why the villains didn't team up very much.

Arkham inmates don't like each other enough to put aside their differences for unspecified time needed to kill the Bat. (Hell Spider-man's Sinister 6 don't even like each other, but at least they have better team-work skills than Batman villains.) Two-face and Penguin are crime boss rivals. Ivy despises Joker. Hardly anyone took Riddler seriously. And I don't think any of the other villains would trust Joker since he has a habit of betraying every villain he's ever worked with.

Pretty much all Batman villains feel that it's their right to kill him to boost their street cred. Killing him as part of a team is no good. Joker even mourned Batman's "death" for a few seconds in The Man Who Killed Batman.



Its amazing how people hang around message boards of movies they dont like

It was more likely something a bit more boring like the budget didn't extend to getting more than one guest star level voice actor. There are only a handful of instances with teamups. Almost Got Im probably being the most memorable.