Other mistake: Right after Otachi sprays acid, Gipsy reaches in its mouth to pull its tongue out, but doesn't get affected by any.
Factual error: When the fisherman in the beginning locates the supposed island on the radar he says it's three miles away. The Kaiju (island) then crosses the distance to the boat in about ten seconds. This would mean that the Kaiju swims at a whopping 1080mph (1738 kph) which equates to about Mach 1.4.
Continuity mistake: The crew of Striker shoots Leatherback in the eye, but in the next scene, it's fine.
Continuity mistake: When Chuck Hansen is sitting at the dinner table arguing with Raleigh he is bareheaded. He stands up and in the next shot he has his baseball cap on. He had no time to put it on at all. It seems to have spontaneously appeared on his head.
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