The following article contains spoilers from Batman/Superman: World's Finest #9, on sale now
In Batman/Superman: World's Finest #9 (by Mark Waid, Dan Mora, Tamra Bonvillain, and Steve Wands) Boy Thunder continued his heroic training under Batman, learning about the different types of villains in certain parts of the world. In particular, Batman made an interesting about the difference between Gotham and Metropolis. While Gotham's villains tend to skew darker and more depraved, Metropolis is a beacon for mad scientists and world destroyers, something he could never understand.
The main issue is that Batman cannot figure out what draws those kinds of villains to Metropolis. Gotham has always been a dangerous place, long before Batman showed up, but Metropolis is one of the bright spots in the world, yet it tends to attract the most destructive villains. The answer might actually be Batman's partner: Superman. Ever since he arrived in the city, his presence invites more dangerous villains to match him.
Superman's Metropolis Criminals Are Different From Batman's Gotham
Batman isn't wrong, Metropolis does have dangerous criminals, but their threat comes in the form of how much damage they can do to the city as a whole, not how sick and individualistic each crime is like in Gotham. Yet, it always falls under the categories of science criminals who invent weapons of mass destruction, such as Lex Luthor, or beings whose power threatens to destroy the planet, like Darkseid or Doomsday. However, villains of this nature almost always targets Superman, either for personal reasons or because they know he will become an obstacle for them regardless of what they do.
In essence, it boils down to the idea that super crime in Metropolis only exists because of Superman, and focuses on him. Normal criminals don't stand a chance against him. Sure, ordinary crimes happen all the time, muggings, bank robberies, murders, but stopping these lawbreakers is often the opening page or two for a Superman book before heading into the real plot. A reminder that Superman can handle these people easily. As a result, his presence in the city invites a greater challenge to compete.
Superman Is To Blame For Metropolis' Villains
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This explains why some of the DCU's most impressive individuals are on the roster of Superman's villains gallery. One of the universe's smartest men is his greatest rival, a world conquering god is his most powerful villain, and he has a host of other enemies all of whom could easily level a city. That's because it is the only way someone can actually challenge Superman; by not being afraid of the collateral damage. Unfortunately, this means that Superman is indirectly responsible for all the villains who go on a rampage through the city.
Batman not understanding why Metropolis attracts so many dangerous villains suddenly makes sense: because the obvious answer would mean informing his best friend that he is the cause of his city's woes. Neither of them would want to accept that, and for better or worse these people are here now. Superman going away would not force them into retirement, so he must continue his mission to protect the city. All the while knowing in the back of his mind that the next time a new supervillain appears, there is a good chance that they were inspired to do so by Superman.