system
A chat on subreddit /r/NoStupidQuestions
human
As a European, I would like to put the outsider’s perspective on this, since I assume that those who use 1800 as the metric are mostly American. 

Americans seem to view the founding fathers as some sort of demigods. They are treated as pillars of virtue, they are quoted in speeches about liberty and equality. Americans revere them to the point where they insist on using an outdated constitution that was never intended to be used for long, just because it was written by the founders of the country. Yet those founders were to a large extent slave owners. Being a slave owners more or less automatically includes being a rapist. It includes being responsible for torture. For all intents and purposes, being a slave owner means that you are the scum of the earth according to today’s standard. Of course that is hard to reconcile with the demigod worship. 

Also, for Americans the civil war wasn’t that long ago. If you are a white American with roots in the south dating back 160 years, your ancestors were withers slave owners or benefited from the slave system. If you are African American with roots 160 years back, your ancestors were slaves. Some of those people can be seen on photographs. We can research their names. And it’s very possible that your neighbor’s ancestor owned your own ancestor. If so there are probably blood ties as well. It so close that it can be felt in a way slavery in other places and times can’t. Most Americans have a closer and more living relationship with American slavery than they do with modern slavery in Dubai and Libya because of that. 

American racial divisions also date back to slavery. Most Americans here will have grandparents who lived in the USA when it was an apartheid society. You like to use fancier words for it, but fancy words don’t change facts. The racial economic divisions in modern USA are based in slavery and apartheid. Basically every American alive today is affected by American slavery. The funding system for schools is set up in a way that reinforces this. The amount of wear and tear of your desk in fifth grade was affected by slavery and apartheid. For a country that likes to call itself the land of the free, that must hurt. So of course people wonder why people in the 1800’s didn’t see the problem with what they did.
gpt
Something like 2% of Americans owned slaves.

The very nature of a slave system is such that everyone who isn't enslaved benefits from slavery 

I do dig most of your thinking though- I hardly think it's fair to judge any past society by today's metrics, because under scrutiny they would all fail the moral test. It's best to compare them to others if their times, where they came from and where they went 


I also think you might take interest in some of the private correspondences of some of the founding fathers of America, specifically where their private statements and public positions were mutually exclusive on the topic of slavery. There's a couple of letters and names are escaping me hard right now where one of them is actually talking to someone about how they can't propose abolition even though they support it because the people they represent would lose their shit if they did (not hero worship or using today's standard---- i certainly found it interesting to read their own words about how they seemed to feel stuck between a rock and a hard place on this very issue) 

I apologize if I've messed up grammar or left thoughts half formed- I am very tired