The Bible does not seem to prohibit slavery (whether using the modern definition or an alternate like "indentured servitude"). Does the Bible explain why it does not prohibit these practices?
Anything can be harmful. If you're going to talk about something being harmful, you have to keep this in mind and ask whether it is more harmful or less harmful than the alternatives.
If you actually study the Mosaic laws on the subject of slavery, a couple things become apparent.
First, that indentured servitude under the Law was a contract that a person could enter into voluntarily in order to work off debts that they could not pay. (What are the alternatives? Take a look at the rest of the world. Depending on where and when you look at, they tend to involve prison, extreme physical punishments up to and including death, exceptionally harsh and inhumane consequences for one's family, or some form of slavery.) The evils of slave capturing and slave trading were completely absent from the law.
Second, the bulk of the laws regarding slavery were on one point: defining and protecting the rights of such a servant. For example, it was not legal to separate a slave from their family (unlike in the American South,) nor was it legal to physically abuse a slave. The punishment for doing so was for the slave to be immediately freed, which was equivalent to the debt being annulled. As the primary reason for the use of slave labor has always been economic in nature, (you don't go buying slaves just because you wake up one day, give your mustache a good twirl, and get the urge to go out and oppress someone) this legal protection provided quite a strong deterrent!
Our brains operate on association and analogy. When we see things that are similar, we tend to lump them together by nature. It works really well, right up until we run into things that look superficially similar but are in reality completely different. And the indentured servitude in the Old Testament was completely different from the horrific evils perpetrated by slave holders in the American South or by human traffickers today. Trying to equate the two is a mistake.
While the Bible does not, to my knowlege, prohibit slavery, it certainly does condemn it:
We also know that law is made not for the righteous but for lawbreakers and rebels, the ungodly and sinful, the unholy and irreligious; for those who kill their fathers or mothers, for murderers, for adulterers and perverts, for slave traders and liars and perjurers—and for whatever else is contrary to the sound doctrine 1that conforms to the glorious gospel of the blessed God, which he entrusted to me.
Regarding those two highlighted words, slave traders, it is only used here in Scripture. Scholars differ as to whether it is referring to making people a slave by force or trading in slaves. See this link for some commentaries on this.
However, many people raise the question you have asked. Perhaps this is because other translations use different (possibly less clear) terminology, e.g. the Amplified uses kidnappers and the KJV uses menstealers.
One caveat: when considering slavery in the spiritual sense, it is important to consider who is the slave owner. Paul describes this in detail in Romans 6, saying that we should not be slaves to sin but that we should be slaves to righteousness.