Other mistake: In the scene from Mako's memory, the terrified little girl stops crying and smiles when Pentecost emerges from the Jaeger's head/cockpit. But if the Jaegers are 250 feet tall, how could she possibly have seen him? It's not kneeling when she looks up at him.
Other mistake: Near the end of the movie, Mako swims to the unconscious Raleigh's survival, blows the top shield of his survival pod, removes his helmet and checks for a carotid pulse...while wearing gloves. No wonder she thought he was dead.
Other mistake: Right after Otachi sprays acid, Gipsy reaches in its mouth to pull its tongue out, but doesn't get affected by any.
Other mistake: During the triple event, different war clocks times are different. One that is stopped and the one that wasn't reset.
Other mistake: When Crimson Typhoon's head is pulled off, their location should not be visible on the radar behind Stacker.
Other mistake: The war clock during the triple event does not reset as it should when a Kaiju breaches.
Chosen answer: Because they are scouts for an invasion of Earth from another dimension. They are testing our response to their arrival and military capabilities and the best way to attract military attention is to attack population centers.
Grumpy Scot