Slavery was a norm for all of human society around the world for all of history. Native American tribes had slaves, Africans had slaves, China had slavery.
It was only the Christian West that started to question how the implicit spiritual equality of all people brought slavery into question in the 1700s.
For the Founding Fathers, they were revolutionary simply for saying no to royalty, even the average farmer could and should be in politics. When the lower classes aren't considered equal around the world to upper class men, of course they could see slaves as outside of such debate.
I'll note that it was only white Christian men saying all men were equal that started to end slavery worldwide. But that was a century after Jefferson.
And Muslim ISIS and Boko Haram bringing it back per the Koran is a step backward in the world.