There are successful visual programming languages, like GRAFCET. But it doesn't scale beyond simple problems. As others have pointed out, it's about information

There are successful visual programming languages, like GRAFCET. But it doesn't scale beyond simple problems. As others have pointed out, it's about information density; what can be shown on a single screen, and also inputting data with a keyboard as opposed to a mouse. In principle, though, nothing prevents you in theory from writing very complex programs using visual languages. It doesn't really make things simpler; once you reach that point, as said above it's just more efficient to combine words than drawing shapes with a mouse.