Generally, levels of social stratification last from one generation to the next. While there may be opportunities for mobility, either upward or downward, access to key factors, such as quality educational systems, recreational programs, accelerated instruction and the advantages that go along with personal connections are more available to children in higher levels of society than those in lower ones. This difference in access helps the social strata remain in place.
The differences between social groups are often based on the attitudes that members of those groups feel. Many members of privileged groups feel a sense of entitlement and also believe that members of the lower groups could move upward if they only had a little more work ethic and drive. Whether the dividing line between groups is based on gender, ethnicity or religion, those in the upper strata tend to possess preconceived notions about those below them that help keep the divisions in place.
There are no truly egalitarian societies. Even the utopian attempts in the 19th century and the communist attempts in the 20th and 21st centuries failed in their aims for total equality. The utopian colonies, by and large, fell apart because of the chaos involved with having no one in charge or because the community bristled against the tactics of the leaders. The communist nations that have persisted into the 21st century have tight governmental control over everything from speech to commercial production.
As Dr. Seuss' book "The Sneetches and Other Stories" suggests, even if there were no visible differences among people, humans would find a way to separate themselves into groups. This impulse is part of human nature and has appeared throughout all eras of human history.