That is what we like to think, but whether or not you are a doctor, or a high earning software developer, or a janitor is greatly influenced by the parents you have, which country you were born in, what year you were born in, which school district you went to school in, etc.
Hell, as I grew older, I realized that families in my own tribe that had more male children were generally far more economically successful than the families with more female children (because it was a paternalistic tribe at the time of immigration and wealth buildup in the new country). The men stay together and support each other, the women get married out and go to live with their spouse’s family. Their children’s economic opportunities were most definitely influenced by whether or not their dad had a lot of brothers to depend on or not.
If the income/wealth gap in society is large, there is basically no way to that society does not get stratified. The only solution to excess tribalism is to reduce that income/wealth gap, which is unfortunately moving in the opposite direction.
Note that I am not a proponent of a caste system or a formal method of tribalism, but I am merely pointing out that marrying someone in your own tribe is expected behavior, and will still happen absent a formal and rigid tribal delineation.