Talk politics.

Actually very few of the slaves did. The USA was not exceptional in this regard, there were regular bustings of slave conspiracies and several large-scale rebellions. Similarly, if slaves *had* been content in slavery the North would never have been able to recruit 200,000 Southern blacks into the USCT units in the war. And this applied everywhere else, too. In countries of the modern world that picked slaves from majority-Muslim groups, the ability to read and write led to rebellions on a scale that dwarfed the Nat Turner Rebellion.

Slaves were given those keys in the US Civil War, or more accurately slaves forced the keys to be proposed to be given and then ensured they were given and kept.