They walk over each other too. We can't do that without causing minor or major injuries
People like to marvel at ants by taking everything they do and scaling it up. That’s not a valid thing to do, physically speaking. Kinetic energy is proportional to mass times velocity squared. Compared to humans, ants with negligible mass and very low velocity have minuscule kinetic energy when they collide.
;-) Sure, not as fast as humans, but seeing something small (and potentially with big poisonous pincers) move at that speed is a little freaky.
"To the mouse and any smaller animal [gravity] presents practically no dangers. You can drop a mouse down a thousand-yard mine shaft; and, on arriving at the bottom, it gets a slight shock and walks away. A rat is killed, a man is broken, a horse splashes. For the resistance presented to movement by the air is proportional to the surface of the moving object."