I mean really now, if this was about ending slavery, why tolerate slavery in four northern states?
Because they didn't rebel. If the south hadn't rebelled, they could have kept their slaves, too.
And spare me Missouri "ending slavery on it's own."
So you're arguing that Missouri didn't end slavery on its own before the 13th amendment was passed?
Mind you, not one of their congressional delegates signed anything to end slavery until the end of the Civil War.
What do congressional delegates have to do with ending slavery within a state? Maryland enacted a new state constitution in November, 1864 which ended slavery there.