There's a minimum amount of energy required for the process to happen. Below that, it can not. Above it, things can happen but how often depends on the intensit

There's a minimum amount of energy required for the process to happen. Below that, it can not. Above it, things can happen but how often depends on the intensity of the light. Increasing intensity will make it happen a lot more often. Some parts will receive far more light then others, making the polymerization progress far enough to actually build a solid object while the rest of the volume is illuminated little enough for the resin to basically stay liquid. (Remember that you need to shine into the container, therefore illuminating liquid that you do not actually want to polymerize but need to shine through to reach the material that should. A sort of collateral-damage if you like). The remaining liquid material is removed but will still contain partially polymerized material that's not grown large enough to actually behave like a solid.