Enslaved people tell us again and again that slavery is horrific in just about every possible way. The violence, including sexual violence, the overwork, torture, scanty provisions (sometimes literally starvation rations and/or stuff too nasty for whites to eat) and conditions all factor in. But what they found most personally traumatic was the destruction of their families. At any moment, your spouse, children, parents, anybody you loved could be sold away from you. Unless they're moved somewhere quite near to you, you may never see those people again. They're as good as dead, as are you to them.

Considering that family is just about all an enslaved person really can hope to have, the loss of those loved ones packed an extra punch on top of ordinary bereavement. That's a hard thing to turn away from. As a result, most successful self-stealing slaves seem to have been relatively young and unattached men. You do see some married couples, women, and small groups, but it's harder for many people to get away at once than just a few right up until the times when US armies are near. During major disruptions of the system all that changes, most famously when the US armies did show up in the 1860s, but also with the British Royal Marines raided the US coast during the War of 1812. They were happy to take on enslaved scouts, promising them freedom for service, but those scouts developed the habit of scouting their new buddies right back home to liberate their families. Their officers weren't thrilled with the extra people to take care of, but found ways to make it work.

And what if you fail? Running is extremely dangerous. If caught, slaves can expect harsh punishment. A beating is probably the start of it, but "troublesome" slaves could be sold as punishment. Then you get the bereavement, plus a whole boatload of unknown dangers at the far end of the sale, without any freedom. If anything, an enslaved person would be sold to somewhere where escape is harder still. Failing that, they might be badly hurt or killed in the course of recapture. Slavery's unimaginably awful, but as long as you stay put you might at least have your family until the enslaver changes his or her mind. It's a dangerous gamble.

Then factor in the distance. It's a lot easier to flee near to a free jurisdiction than from the heart of the Cotton Kingdom. People do get out from the Lower South, but they're so rare that it's not really a political issue for the white establishment the way it is further north. (Even there the scale isn't massive, though it is enough to make it a political issue.) They stow away on trains and steamboats where possible, but a lot of running away is going to be on foot, at night, and through increasingly unfamiliar territory with limited provisions and over land that whites patrol (sometimes formally, sometimes just by keeping an eye out as they go about their business). All of those whites have the legal right to stop and question a black person and to use force to ensure compliance if refused.

As a result, a lot of those enslaved people who do bolt probably don't expect to get freedom so much as a few days of relief. It generally went much easier on slaves who would take their time, maybe visit someone a ways off who was important to them, and then return on their own. Punishment is still very likely, but an enslaved person could reasonably hope for less severity.

But say the enslaved person gets across the right state line and hooks up with free black and white antislavery communities. That's good so far as it goes, but the US constitution grants enslavers the right to recover their fled property. Being in a free state doesn't necessarily make you free (it can, but not for runaways) and your ability to remain is dependent on the community's willingness and ability to protect you. That's another gamble. Many slaves who make it that far end up fine, but some do not. At any time the mood might shift against fugitive slaves and someone could come calling for you, your children, etc. True safety and security are possible, especially with a large antislavery community about, but to get a guarantee you'd have to go all the way to Canada. That's doubly true after the Fugitive Slave Act of 1850 becomes law.

It's easy for us to say that they should have fled but escape is a dangerous choice with massive consequences. How many of us want to kill everyone we care about, or kill ourselves and leave them with the grief? Running away is functionally the same, unless an enslaved person gets very lucky. And that if it succeeds. Failure risks all of that plus a strong chance for some extra torture. It's an impossible situation.

Sources

The Slaveholding Republic by Don Fehrenbacher

River of Dark Dreams and Soul by Soul by Walter Johnson

The Internal Enemy by Alan Taylor