In short, generations exist because cultures change. Just as Japan has a different culture from the U.S., the culture of the 1950s was different from the culture of the 2010s. As cultures change, younger people – who have never known another world – take certain attitudes and worldviews for granted. For example, most Americans now These specific changes often stem from broad, pervasive forces in the culture.
One of these pervasive forces is individualism. As noted above, individualism is at the root of the movement toward equality based on gender, race, and sexual orientation, and also encourages positive self-views, uniqueness, and high expectations.
Another cultural force is technology. For example, iGen has been shaped by their adolescence spent on smartphones. A generational shift appeared around 2011-2012 among teens, likely because smartphones became pervasive around that time.
Last, families are smaller and lives are longer, leading to more people pursuing what is known as a slow life strategy. Teens who pursue a slow life strategy, for example, will be less likely to engage in adult activities such as driving, working, dating, drinking alcohol, having sex, and going out without their parents. Some of these trends are good, some are neutral, and some might be bad – but all involve growing up more slowly. The theory behind slow life strategies (life history theory) explicitly notes that slow (or fast) strategies are not bad or good – they are an adaptation to a cultural context.
So, although generational labels are not always precise, it is very clear that cultures change over time, and that those changes have an effect on people. That, at base, is why generational differences exist.