Because that is basically what you have to do if you want a truly rich editor that works and looks consistently across browsers and operating systems. Maybe to

Because that is basically what you have to do if you want a truly rich editor that works and looks consistently across browsers and operating systems. Maybe today you could get something to work using a combination of contenteditable and canvas, with code to handle the inconsistencies between browsers. But I know that at least a few years ago, rendering to a canvas was a necessity for rich web apps like this. Google docs at least used to be canvas based as well. I'm not sure if that is still the case.