Simple answer - because you cannot compromise the worthiness of one's home. A sloppy painting job can switch off even the absolute most interested buyer and make your home's value plummet significantly. Yes, it's admirable to might like to do the whole lot yourself and not need to cover professional painters to complete something you've probably done before. But here is why hiring a contractor would inevitably be the higher decision on your part.
To start with, painters are specially trained individuals who have experience in a number of different projects. The common homeowner probably has some basic house painting skills, but very few of these would measure as much as somebody who actually paints for a living. And I'm sure you'll agree with me that you, as a homeowner, have better things to complete than spend the entire day painting your house, whatever the reason. You'd favour someone get it done for you while you focus on other matters.
Secondly, painters will ensure that your house has the proper finish that would protect it from wear and tear. A sub-par painting job won't do much to defend your property against water damage, mold, rust and other external factors that may decrease its value. You'd want your property to last provided that possible, and having the good qualities do the painting for you personally is one method to boost the chances with this happening.
The third and fourth primary reasons to hire professional painters will be convenience and safety. Returning to the point we made earlier, it could be tiring to complete the painting yourself. And we haven't even mentioned the chances of having your clothing permanently stained or the possible injuries that may occur should you be working with ladders and painting the next story of one's home.