There are still hierarchies. Both male and female hierarchies are present in primates. It's not alpha and the rest, it's the whole alphabet and each level dominates the next.

Whenever the hierarchies are unestable primates can behave very violently. Hierarchies preserve status quo and reduce violence.

Also, the studies done with Bonobos were flawed as well, because they were only studied in captivity. Bonobos when free are only a bit less aggressive than chimpanzees, and they don't avoid all violence by using sex.