why does nick call tom and daisy "careless people"?

Daisy and Tom Buchanan are "careless people" because they are insensitive and thoughtless. Tom physically abuses Myrtle Wilson by striking her in the face and leads her to believe he would marry her if his wife weren't Catholic, which is a lie. He misleads Tom Wilson by implying that Myrtle was Gatsby's mistress and that Gatsby was responsible for her death. Thus Tom gets Gatsby killed, but Daisy is the one who was driving the car that struck Myrtle. Daisy, of course, never admits her guilt.

Daisy enjoys flirting with Gatsby, but when problems arise, she hides behind Tom. Somehow she doesn't foresee the confrontation that takes place between Tom and Gatsby in the hotel room, perhaps because she hasn't been taking Gatsby's attention seriously. This couple tends to run away from difficulties, mostly because they have the money to do so and they have little sense of obligation to others. According to Nick, they "smashed up other people's lives" and left the wreckage behind for someone else to clean up, someone like Nick, who does have a sense of integrity and honor.