In reality slavery has existed long before America was even dreamed of, and still exists in other countries, so why do both our society and education system pretend that black people are the only ones who have ever been enslaved?
Although I can't speak to contemporary slavery practices which of course does exist, I can perhaps try to answer your question. There are major distinctions between slavery in the ancient world and modern racialized chattel slavery, which is perhaps why it is emphasised in the US education system as racialized chattel slavery is directly relevant to the founding and building of the United States. A few differences:
Something that Orlando Patterson calls "social death". Which means there was a chance for enslaved people in ancient Rome to be integrated into the community, or even into a family. However, enslaved peoples in the Atlantic slave trade were selected because of their "racial" difference, which made it impossible for them to assimilate into their new forced societies. Their families were broken apart and they were removed from their communities and forced into societies that did not legally allow them to participate, hence the term "social death," since enslaved peoples from Africa did not have any social support systems. In fact, black skin became a signifier of a slave. If you were black in America it was assumed you were a slave and you had to carry official documents to prove otherwise.
The invented concept of race as biological difference was of course not a factor in the ancient world. Was there an idea of differences, yes, but it was not until the Enlightenment that these "racial" differences were supported by "science." And it was not until the Enlightenment that peoples from Africa were seen as, in some cases, subhuman. All of this was justified by scientific inquiry. Here, again, this made it impossible for peoples from Africa to assimilate into their new forced societies. So complex race theories were invented to support the practice of enslaving only peoples from Africa and not other places.
Chattel slavery was quite different from obtaining slaves from "just wars" in ancient Rome. Slaves from war come from certain types of social structures, and were predicated on war and annexation of territories. Chattel slavery had built around it very complex social structures to support the kidnapping, and selling of specifically peoples from Africa, supported by race ideologies that claimed peoples from Africa as mentally inferior but physically strong. Chattel slavery was also supported by complex laws that legally codified enslaved peoples from Africa as property within America's growing legal culture, which is why you see slavery in founding documents of American history like the Constitution. For reasons like the history of the law in the US racialized chattel slavery is quite pertinent.
Robin Blackburn has a great text called The American Crucible that will answer your question in depth as he goes through the history of slavery from the Ancient west to modern racialized slavery.
So to answer your question more directly I don't think anyone is "pretending" slavery only existed for "black people," but they are talking about a specific type of slavery, that is racialized chattel slavery specific to US history. Perhaps if you take an ancient Rome course they will teach about slavery in a different context there, and I am sure courses in contemporary political issues or even International Development will teach about current systems of slavery.