Jefferson said he'd have freed his slaves if it were legal to do so. He treated them more as employees than slaves. As to why he didn't free them, I don't think you know.
I hardly think holding people against their will is "treating them like employees." Don't think there were too many slaves who thought that way, either. That is why they left southern plantations by the hundreds to get to Union-controlled territory and enlist in the Union Army. Some "employees."