Because that’s not how they work.  It is not “entirely possible” if you actually know how they work.   There is a saying that “any sufficiently advanced technology is indistinguishable from magic”. What that means is that if you don’t understand how a technology works you can be told it is magic and there isn’t a way to tell the difference.  A wheelbarrow has actually been considered magic because the people viewing it didn’t have the background to figure out how it worked.

For the people who know enough about how cameras work the concept of ghost pictures is stupid. 

Film cameras use granules of material sensitive to a given light range.  CCD cameras do  the same but with a sensor.  Those sensors or granules detect radiation within a certain range and react according to it.  That is all. 
 

“Ghost pictures”are generally tricks of peculiarities or faults of the way a given camera is designed.  They are only “ghost pictures” if one doesn’t look deep enough into the given situation. 
 

human technology became complicated enough hundreds of years ago for this sort of trickery to become possible.  Sailing ships that are “pulled along by unseen forces”(true)”beneath the waves!”(false)

an example:

it turns out that humanity actually had how sails work wrong during that time. They made square rig ships. Sails don’t “catch the wind”. If they did sloop rig ships wouldn’t work. It turns out square rig had some advantages for things like turning so they stayed on even after faster and higher pointing designs were created. 
 

There was a time, just pre 1900 when cameras were new, that “ghost pictures” were the rage because cameras were poorly understood.  After cameras became better understood “ghost pictures” were relegated to incorrect understanding. Much like sails “catching the wind”.

”I don’t understand why people don’t think ghost pictures are real” means “I have a pre-20th century understanding of cameras” it’s a bit like that juggalo song “how do magnets work”  there are people who understand how magnets work and they will generally even tell you if you care. Or you can just go about your day worrying about different things and leave the magnet people to do their thing.