Because domains need to be registered to work on a Global level. Somebody already owns img.com and you would need to persuade them to give up the rights to it. People who already have short domains like 3 letters long know they are sitting on a potential goldmine if some entity wants to purchase the rights.
Part of why they are valuable is they are easy to remember and quick to type which lowers the friction for entry. When you want to do huge business things that might prevent a small percentage of people from visiting your site adds up to huge numbers of lost potential business.
