Lasers do exist in the story, but they're not widely used in warfare because when a laser makes contact with a holtzmann shield (the personal shields that all of the characters employ) it causes a nuclear reaction that annihilates both parties and everyone in a large radius. This makes them extremely risky to use.

There's a great moment in the book where a character sets a trap by engaging a shield and fleeing the area before an errant enemy laser eventually connects with it and creates a massive explosion to cripple the invading forces.

With that in mind the careless use of lasers in the film on two occasions is puzzling considering that it would be an extremely risky and dangerous tactic in the book's universe.