Sometimes it was but that was mostly during the Roman Empire and only for favoured slaves who earned "manumission" many other times it was that they were raped, beaten, killed, used in Colosseums etc. Let's not downplay the badness of Ancient History so that we can appreciate how bad more recent slavery.

Also, slavery in ancient times happened over thousands of years. It was more class-based and war-based though. But the Ancient world was more of a melting pot than slavery-era America. People were slaves usually because they were the descendents of cultures who were conquered. The thing is, the Greeks and Romans etc didn't think of slaves as being a particular colour.

Black slaves in America also were enslaved by white Americans initially due to colonialism, it wasn't so much they were made slaves because they were black, they were enslaved by their neighbouring tribes and sold to the white colonisers, who I believe would have bought them and used them as slaves even if they were a different colour to black. Their black skin became a post-hoc rationalisation for why they "deserved" their fate, in the minds of the white slave owners. And then once they were IN America they stood out amongst the white settlers, and their skin marked them as the class of "slave". And that's how being black and being a slave was so entrenched in the mind of white America.

I think this is also where certain racial concepts became important to the national psyche, because the slave owners needed a way to cope with the cognitive dissonance that would invariably come from having people as property. So they could comfort themselves thinking "they're not really people like us so it's not immoral, God/evolution shows they're lesser because they're a different colour". And thus they could dehumanise the slaves with that excuse.

Whereas the slavery in Ancient times, the rationalisation that was used was usually "they belonged to a different tribe/they were our enemy and we defeated them/the gods didn't favour them/ they did something wrong in a past life to be born into slavery so this is their fate and I shouldn't feel guilty".

It's a unique story, but while one should never downplay how horrific the trans-Atlantic slave trade was, we shouldn't downplay historical slavery from societies we glorify now like Rome, Greece, Egypt, Persia etc. Just because they weren't enslaved because of their skin colour doesn't make the slavery any better.

It's wrong to say "slavery has existed throughout history therefore what happened to black people in the trans-atlantic slave trade wasn't unique or that bad".

It's also wrong to say "slavery against black people was worse than any slavery that has ever been".

What's true is that slavery against black people has unique lasting effects on them as a race that doesn't affect the descendants of slaves from more ancient times. Part of it is that they are much more visible as having that history as it's literally on their skin, but part of it's also that slavery against black people was way more recent than the slavery from Ancient times.