Why do many people think the Chinese are traditionally "conservative" and "introverted"?

Because the Chinese emphasize etiquette. Although many people are bold and free-spirited, but in the feudal era, we must abide by the way of the king and ministers, loyal to the king and love the country, and to do their duty. What you said about conservative and introverted character is only on the surface, but more importantly, this kind of people are very observant of the rules, etiquette, and behave themselves, and do not go to do something out of the ordinary. So they are more traditional.