Why were blacks not recognized as humans during the days of slavery? Were they unable to put two and two together and see the similarities?

They were seen as human.  Obviously, black people are human.  But white people saw them as a lesser form of human and one which had fewer rights.

In order to treat people unfairly or to do mean things to them, evil people have to vilify their victims. For example, the fascists want to make the rich richer and the poor poorer. So, to do that, they claim that poor people are either lazy or dumb and therefore they do not deserve help. Trump wants to exclude Hispanics from this country, so he claims that they are drug smugglers, rapists and murderers. If slaves are regarded as subhuman, then it is okay to treat them like animals. Hitler for example called the Jews "cockroaches" and then he worked them to death without feeding them and put them in gas chambers. What did neo-Nazis say in their Charlottesville march? They said that "Jews will never replace us." Jews are of course not out to replace anybody. They are here to live peacefully as Americans. But in order to do mean things to good people, evil people have to smear the good people first. Trump is trying to smear Joe Biden. Republicans have been smearing Hillary Clinton for years. They were chanting "lock her up" in 2016. We do not lock good people up, so they need to smear her before they can chant "lock her up."

It wasn't just black people, in a lot of countries, there are a lot of same ethnicity slavery systems too, they sure know all are humans, and they sure see how similiar their appearances are, but they would call it status difference. Back then, people don't see each other as equal, a lot of times, a lot of them only see family status.

There are still plenty of racist people today who consider Blacks inferior.
America was founded on slavery. It became intrinsically woven into the fabric of economics. MANY people willingly accepted "conventional wisdom" to conclude Blacks a sub species to fit profiteering. It's a classic case of hypocrisy ... do as I say, not what I do. Look how many had sex w/ Black women but she was a sub species.

That attitude was taken by all of the countries who colonized Africa and first settlers in America came here with those attitudes.

Slavery didn't start with the African slave trade. Slavery has always been a thing. We just ended it.