Why? Bible tells that person who kidnaps other must be killed.
Roman law said that as well.
Was there no slavery in Rome?
That obviously means, one can become a slave only voluntarily
No, it doesn't mean that at all.
Obvious examples are buying slaves from other nations (it is not known how they became slaves), blood slavery (if an israelite slave has children while being a slave, the children are automatically slaves as well) and prisoners of war (which doesn't count as "kidnapping").
I agree, but it makes it not possible to get a slave by force.
If you agree, then the conversation is over and you concede the point, since your point was that there is no actual slavery in the bible "because you can't kidnap people".
Again, in ancient Rome you couldn't just kidnap free people and enslave them either.
But I bet you wouldn't dare to use that as an argument that there was no "real slavery" in Rome, right?
I don't think you'll find a society that allowed slavery which didn't have laws against just kidnapping free people and enslave them.