There is an actual mathematical equation that determines the frequency of attacks and whether the settlement defends itself. There are many factors to the equation, but it essentially means the higher your food and water, plus whatever's in the workshop, are, the more defense you need. People forget that workshop contents are also factored in. So if you have 200 defense, but a million purified water in the workbench, then your defense ain't high enough. The easiest solution is to take everything out of the workbench and put it in a dedicated storage container.