because they think they can do bad things without getting caught. cocky attitude. stupid excuses that defend their decision to cheat. basically they are idiots who should get cheated once or twice and see how it feels.
People are naturally evil, selfish, & inconsiderate. In some cases this is learned, from parents or others. Public opinion favors instant gratification, "can't help it" blame shifting, evil perversions not tolerated a couple generations ago. As the Bible, God & morales are relegated to being "old fashioned" or "silly" rather than RIGHT & PROPER ways to live one's life, this will only get worse...
First you're asking a question in which none of the answers will make you happy so don't get mad at me... Many reasons cause people to cheat. Insecurity, lack of sexual satisfaction, lack of emotional satisfaction, falling out of love with your significant other or even falling in love with someone else. People will even do it to get revenge. Girls will often cheat on the first guy they were with to see what a sex is like with someone else. Guys are actually genetically programmed to be promiscuous like males of most species because it helps spread their own genes and increase the population which furthers the species. That urge can be easily suppressed however as I've been completely faithful in all relationships including my current one of 12 years.
90 percent people are greedy and stay connected for just their own stength and benefit and just care about themselves. but they dont know that they have to care about others too in order to care themselves.