The interesting thing is that it is in a human when they are born, grows until puberty, then gets smaller and smaller until it can be quite small and difficult to detect in a grown adult.
I can imagine medical explorers cutting open dead 40 year olds in the year 1900, probably not finding any obvious organ there -- while perhaps cutting open dead children may have been a lot less common (and perhaps distasteful). If you did find something there, you would not assume an important organ present in a child and essential for their immune system would shrink and almost go away.
It would be more likely to be labelled nothing, an abnormal growth, or even a cause for death or illness (pressure on the heart/lungs!).
Woody Allen's second favorite organ is the brain : https://www.youtube.com/watch?v=ngizj5FIcjo
"No shadowy Illuminati group deliberately made this decision, but as a civilization we have collectively ‘decided’ that three groups of people would get to peer inside the human body, and they’d all be hobbled."
Like the guy who built some of the first microscopes and discovered the sperm in semen, he wrote to the scientific community to discuss the discovery, but made sure to include that the semen he used was "what was left" after he copulated with his wife. Wouldn't want to give anyone the idea that he had masturbated, heaven forfend!
You know, diversity improves results.