Nick refers to Tom and Daisy Buchanan as careless people because he's witnessed them ruin lives and simply disappear by hiding behind their wealth. Throughout the course of the summer, Nick Carraway witnessed Tom Buchanan manipulate George Wilson and carry on an affair with his wife behind his back. Tom displayed a complete lack of sympathy for George and was simply using him so that he could communicate with his wife on a regular basis. Similarly, Nick witnessed Daisy carry on an affair with Gatsby and play games with his heart. Once Daisy learned that Gatsby was a bootlegger, she went back to Tom because he was financially secure.
After Tom exposes Gatsby as a criminal, Daisy drives Gatsby's car and runs over Myrtle Wilson when she sprints into the street. Daisy then demonstrates her selfishness by allowing Gatsby to take the blame for the murder. Furthermore, Tom tells George that Gatsby is responsible for his wife's death and George proceeds to kill Gatsby before committing suicide. Nick recognizes that Tom and Daisy are directly and indirectly responsible for Myrtle, Gatsby, and George's deaths. Despite their actions and prominent role in several deaths, Tom and Daisy refuse to take any responsibility and leave the East Coast. At the end of the novel, Nick runs into Tom and mentions,
They were careless people, Tom and Daisy—they smashed up things and creatures and then retreated back into their money or their vast carelessness or whatever it was that kept them together, and let other people clean up the mess they had made... (Fitzgerald, 191).
Nick's comments stem from witnessing Tom and Daisy's selfish, impetuous nature as they make a mess, ruin lives, and hide behind their wealth. Daisy does not even attend Gatsby's funeral and Tom shows no remorse for his actions, which underscores Nick's critical evaluation of the couple.