Probably because it's metaphorical and not actually about the penis.

Now that I have this giant truck, I own the road, I can look down on everyone, no one can laugh at my personal faults and failings.

Idea that the truck can be used as a prosthetic for the personality is funny but hard to boil down boil down to 4 or 5 words. Male genitals however are euphemistically used all the time to describe (mostly) positive aspects of people. "Big dick" energy, "the balls" to do something, so the quickest way to express that a person is compensating for a lack of something that they know about and are ashamed of, but are unwilling deal with, is a tiny dick joke.

Thank you for coming to my Ted Talk.

edit: missed a word