Humans are social animals, and humans brains are wired to work in concert with those around them. Social pressures are also strong, and many people have a strong desire to fit it. Conformity has evolutionary advantages that helped early humans.
Historically, humans have needed to get along and work together to survive. Humans used to live nomadic lifestyles, and they had to rely on each other to get food, find water and construct shelter. After human started settling down, they formed small communities where everyone had to help. The natural instinct to conform likely helped early people survive and thrive. Studies have shown that the desire to conform is partially hard-wired.
However, there are social reasons why people conform as well. Many people value being accepted, and conforming to an extent is helpful. In addition, most people have a natural aversion to conflicts. By at least tacitly conforming, people can ensure they are not constantly engaging in arguments.
Some people pride themselves on being non-conformists, and many people fight against conforming during adolescence. However, young people typically outgrow this phase, and many eventually realize that conforming to social norms and customs helps them build friendships and avoid unnecessary conflicts as adults.