There are companies selling items that must be assembled by the purchaser, and some of them have turned to pictures and videos to help their customers. This sounds like a great idea, but a picture worth a thousand words is useless if it does not show what the assembler needs to know. The ability to read a set of directions is one way to make the project easier, so those who can read have a better chance of realizing their finished product with few errors or missteps.
When videos are made, there is often an assumption that a person has a certain skill level. Written directions are usually closely edited, and they often have extra instruction for those new to assembly. Showing a flat head screwdriver is great in a video, but it can be a missed opportunity if the shot is not large enough. The written directions must tell the person what screwdriver they will need, so this is how written directions are often better than videos or even pictures during an important home assembly project.