It's natural for us to see phallic imagery everywhere we go. The legendary psychologist and pioneer of psychoanalysis, Sigmund Freud, told us that men wish to have the phallus and women desire to be the phallus. We build skyscrapers and obelisks and other long and tall monuments in our subliminal tribute to the male anatomy. He would also tell us that we observe these symbols everywhere we go because deep in our psyche, the phallus tugs at these primeval wants and desires that reside in the darkest pockets of our brains.
I would tell you that the reason for this is that dicks are funny. They're ugly and floppy and sometimes big and sometimes small but always dumb and embarrassing. It's almost like the male body exists to be laughed at, and that's fine as long as we don't take ourselves too seriously.