Here are several reasons;
1. It is illegal unless you have been given written permission to do so.
2. Quality can be very poor when not using the high resolution files to print from.
3. Longevity of your images may be severely reduced.
Of course, some photographers do not claim copyright and even release the negatives. The right of a photographer to copyright the photographs has a firm basis in law. A number of copyright infringers have learned the hard way that illegally copying a photographer’s work is a big no-no and cost them alot of money in damages, fines and legal fees! So unless you have written permission, don’t try it – and don’t ask anyone to do it for you.