Here is an explanation from Venerable Luang Suriyabongs which explains why people believe in karma. Read and contemplate:
"The Law of karma and rebirth is immensely reasonable. It is the best explanation of the inequalities of men; it explains good and evil. It helps to solve many problems of life. Such as, where do we come from, what are we, where shall we go? It encourages man to practice good, to be tolerant and show loving-kindness and compassion to all sentient life. Above all, the firm belief in karma and rebirth makes man sober, not pessimistic nor over optimistic; it makes him see things in the light of karma: life’s impermanence, its misery, and its non-selfness. It prevents him from doing rash acts which might have bad results in his near future; it prevents him from suffering unduly under the karmic conditions created by his own doing. The right understanding of karma and its implications gradually leads man to perfect awareness of the true facts of life; to the realization of the Four Noble Truths; to the conquest of kilesa, of delusion and of karma itself." Luang Suriyabongs