In a perfect world we could build the design once and have it be done. But as other commenters have pointed out, the design process is full of revisions, which can take longer to implement in code than to edit in a program such as Photoshop (especially big changes).

…also some clients check multiple browsers at or before launch to see, and often report it all back to you in an email not always with kind words… so it’s best to let them know up front either that their will be differences and what they are, or just try your best to get them to match (which is painful)