The reason you purchase a license instead of buying the image outright has to do with the way Copyright Law works.  The license is used as a vehicle to transfer reproduction and editing rights from the creator of the image to another individual or entity.  

The Copyright Act - "Protects photographers by giving the creator of the photograph the exclusive rights to copy, edit and distribute the image by sale or transfer."
"Copyright law protects photography as intellectual property and provides protection for anyone who shoots a picture - not just for professionals.  Licensing is used to purchase permission from the copyright owner (photographer) to the client mainly for reproduction purposes, and some cases to purchase all rights to the photography so the client can use it without restriction.  Usually, to purchase full exclusive rights for the image is very expensive, because you are asking the photographer to give up any future hope of further income down the road." - APA Business Manual  

For this reason, you will not find many photographers willing to give up the full copyright rights to their images.