They kinda do. When a fissile atomic nucleus splits, the daughter nuclei repel each other; they are both positively charged and the strong force is no longer holding them together. So they fly off in opposite directions at a measurable fraction of the speed of light. But they don’t generally get very far, because they are embedded in a solid fuel pellet. They can push their way through a few µm of uranium before they are stopped, bumping into thousands of atoms along the way. That’s really where the heat comes from; all that electrostatic force accelerates them to high velocity, but they dump it all very quickly into the material around them as heat.