The pressure to get good grades in school causes many students to cheat on their homework and tests. If the student is not caught cheating, he or she may even get higher grades. But getting caught cheating may also result in being expelled out of school or university. Plus there are long term consequences of not learning and moral declines. It is a personal decision that one has to make on his or her own.
Parents are most likely be the top reason that many students cheat. If their child does not do well or reach a certain level of expectation, the parent may punish or ground the child. So the kid goes out of way and does cheating or finds other ways to achieve the level that he or she were to be expected from.
A person who copies the work of others, whether it is plagiarizing material from books or using the Internet, or just copying homework from friend or another class mate, ends up not having to work very hard for a passing or even good grade. It's just like getting what they want for not working a bit for it.
Getting caught cheating on a test of copying someone else's material for homework can result in a lowered grade and even expulsion from school. If they don't get caught, there is still the problem that they haven't learned their material. That ignorance will eventually may come to haunt them down the road, when someone is required to apply what they have learned, the person who has been doing good by cheating wouldn't have anything to show or prove.
In either case, there is also the moral stigma that you subconsciously hold inside of you that you didn't deserve the grade you got and that you are a cheater. Such a person may go far in life, but he or she will never be a champion they expected to be in life.
In conclusion students may be pressured by their parents to get good grades, or they may be too lazy to do the work, so they cheat on homework or tests. Although they gain in the short-term with good grades, in the end they lose out.