I'm certainly not saying that Putin keeps borders in place. I'm explaining why borders are used to define the conflict. Otherwise you're taking about a full census and genetic testing to figure out which households are Ukrainian, and thus likely not willing to submit to the force on someone that just showed up from somewhere else wanting to tell people how to live.

Borders came from agriculture. Farmland. Who gets to feed their children the food they grow. Who can graze livestock where. Case in point, herder-farmer conflict is a huge problem across Africa. Because there are no real borders for Fulani herders, or fences for farmers.