Why is sociology NOT considered part of anthropology?

sociology and anthropology share the same roots, and even some of the same founding theorist. Some Universities in the US still have combined sociology and anthropology departments.

The real difference between the two is the scope. Where as sociology tries to explain humanity through social constraints and pathways, anthropology is suppose to create a holistic source of knowledge, taking into account equally all aspects of humanity.

Combining: biology (physical anthropology), language (linguistic anthropology), statistics, the past, and environment (archaeology), culture, sociological, and indigenous knowledge (cultural anthropology) and everything else in between; anthropologist try to construct a multifocal understanding of what it mean to be human.

Sociology could function without anthropology, however anthropology would be severely hindered without the tools and knowledge gained from sociology

Sociology is actually older and a foundation for modern day Anthropology.

Scope is the difference. Sociology studies the societal and social aspects of human beings. Anthropology studies humans. In general. There are 4 basic sub fields of anthropology: biological, archaeology, linguistics, and cultural. The basis of anthropology is holism: studying the whole of the human condition, from our evolutionary past to our current cultural situations. Anthropology and sociology do have a lot in common as far as theoretical basis goes, especially in the late 19th and early 20th century (Durkheim is especially important for theory in both fields). Sociology also focuses on society, as does anthropology, but remember that anthropology also focuses on individuals. Cultural anthropology and sociology do have some commonalities, however it seems to me that one of the main differences is on the focus: societal level vs. societal AND individual level.

Anthropology does not just study people from the past. Anthropology studies people past, present, and future. So does sociology.

Anthropology in the U.S. = culture, biology, linguistics, archaeology.

Sociology = society.

In layman's terms anthropology is the study of humans from the past. Using evidence like fossils, ancient materials like living quarters and tools, genetics, etc., anthropologists try to piece together a picture (what people looked like, how they migrated, how the affected the environment, etc) of life thousands upon thousands of years ago. Sociology is the study of contemporary or near past group dynamics and interactions. Anthropology relies on evidence and concrete proof to form hypothesis and thesis. Sociology still relies on those things, but the results are much more open to interpretation. Consensus is much easier to come by in anthropology than sociology.

I'm guessing you're confusing the fact that anthropology can include sociological elements. But it's a much wider field than mere ancient human interactions that explores ancient technology, group migrations, genetics, etc. Sociology may explore technology and other non human elements of civilization (say the advent of social media) but the endgame always comes back to how such technology affects group interaction.

Both fields study humans so overlap is inevitable.