This is not just because of the impression that it gives, but also due to the things that a prospective buyer can think when seeing a messy home.
One assumption is that pests could be hiding in messy areas of your home. It is virtually impossible to spot a rodent infestation if they have their pick of your home for hiding places.
Another assumption is that there isn’t enough storage space in the home, causing you to store your things out in the open. Even if this isn’t the case, a cluttered home looks smaller than one that is straightened out.