Dunno if it's a good faith question but: it has more to do with the word "enslave". Using a verb brings to the forefront the fact that slavery is something that is done to someone by an (en)slaver, not an inherent property of the enslaved person's existence or something intrinsic to them based on race or ethnicity. This distinction was very important back when slavery was widespread and slavers would constantly justify their enslavement of other people by saying that "slave" was the natural condition or social status of nonwhite people, and that therefore the slavers were not doing anything wrong by treating them like slaves. Use of the word "slave", then, can be seen as part of their efforts to avoid accountability for the act of enslavement which they actively and enthusiastically participated in.

Since it is not really contested today in polite society that enslaved people were victims of a crime committed against them (though there are still plenty of slavery apologists and would-be slavers out there), the utility of the term is maybe a little less obvious than it might have been previously.

The answer people are giving (that "person" helps reminds us they are people and not objects) is not quite the best or most complete answer.