Because Photoshop experts have debunked hundreds of altered pictures, or if the dick is real, the ruler isn't.
If a picture of a real 10" penis was posted, using a real ruler,it would be scrutinized for evidence of manipulation, but if none were found, $10K to $20K is readily available!
And if someone who actually
knew they were some extreme size and had posted a
legit picture started getting torn apart by people calling them a liar and pointing out the flaws, they would simply post one or more less questionable pictures or a video clip to
shut the people up, rather than using the age-old "Fuck all you haters, I'm done with this!" tactic. If someone's walking around all day with the actual
real dick hanging off their body, as convenient as such things are today, it's not some big ordeal that anyone needs to "work on getting" or prepare for. If they say they do, it's because they're working on the picture rather than the taking of it, or they don't feel like doing all the work a second time.