The fact is that the various authors of the Bible could have simply stated that it is wrong for one human being to claim another human being as property. They didn’t. They were products of their culture and time, as are their writings. If they were truly inspired by a god and if that god truly wanted people to think that slavery was wrong, he could have inspired them to say so. However, what we have is exactly what would one should expect of purely human books written by humans who were of that time and of that culture, nothing more.
Now, it’s true that some parts of the New Testament are somewhat progressive (eg. Galations 3:28) but those passages are counter-balanced by others that prescribe an inferior place for women and which instruct slave owners on how to treat their slaves rather than telling them outright that owning people is wrong.