White people did not invent slavery. White people did, however, invent industrial chattel slavery.
For most of human history, slaves were taken from peoples with whom your people went to war with, or they were people who owed debt, or were criminals. These slaves were acknowledged as humans and, like you said, oftentimes were not in slavery for multiple generations as the owners would free them upon death or the slaves' children would be free even if their parents remained slaves.
European racial theory turned that all on it's head. No, racism is not 'I don't like you because you're different". Racism is considering people of another race less than human, or at best, less human than the master race. I've seen you mention other societies with ethnic conflict all over the world. In a modern sense, sure, you could call them racist, but they inherited that logic from white Europeans who once colonized them. In most cases, and especially in the past, those ethnic conflicts were understood as conflicts between two equally human groups of people who hated each other for ideological reasons, not because one group considered the other group to be inherently inferior humans.
So back to slavery - What European racial theory did to slavery was to consider black and other non-white indigenous peoples as inferior humans who were destined to be a commodity for the superior white Europeans. These were not slaves akin to the ancient Roman style of conqueror slavery or debtor/punishment slavery, but were instead more like how we treat livestock.
Slaves in the Atlantic Slave Trade were conquered by other Africans, sold in bulk, shipped, sold at market, and bred and raised for live in forced labor. This is just like how animals are hunted, domesticated, bred, and raised for slaughter.
This form of industrial agriculture slavery was a new phenomenon from the 14-1500s onward and absolutely invented by white Europeans. It had everything to do with the combination of European racial theory and growing European industrial capitalism that didn't really exist anywhere else in the world.