I’d also imagine that the movie stars themselves have a lot of riders in their contracts as to when/if they’ll pose for stills and publicity shots, who has to shoot it* whether or not they approve of this photo or that, retouching, what size each image has to be in comparison to their co-star** and a myriad other pain-in-the-ass factors. It’d be easier just to take a photo that they’ve already done and approved of for some other reason and splice it onto the poster.
As I’ve heard, the makeup used for close-ups is entirely different than what they use for long-shots during filming. Supposedly, some stars do makeup tricks which accentuate certain features that you can’t see from a distance, but would look weird up close. (Like drawing a white line under the eyelid to make the eyes look bigger.)
I’d imagine the lighting would be different, too, which would mean you would have to use different shades of makeup, right?
Lastly, I’d think you’d probably have to have your whole marketing campaign designed in advance. Marketing for a romance is different than it is for a comedy and you’d have to know how you’re going to present the film to the public. Sometimes movie studios slant the marketing to make the movie look more like a different genre in response to feedbacks from focus groups and the like. So, if they wanted to market the film as a romance, they would use romantic lighting, get the actor in a romantic pose, make the hair and makeup fit the genre. Then, the marketing company desides they want to market it more as a romantic comedy, which means a totally different change in look and another photo shoot. Instead, it’s easier just to grab an already-done photo that looks appropriate and slap it in place.
*For some reason, I think I remember a story about a celebrity who had a certain photographer she insisted was the only one who could photograph her.
**I emember seeing a contract rider on Smoking Gun in which the star demanded that her image be a certain percent larger than that of her co-stars.