As BA touched on, Due to patriarchal rule, penises are in fact...everywhere.
The Obelisks in Egypt and now in other countries, were founded on the basis of the design of a penis. Which in many cultures, symbolizes Strength,Masculinity,Virility,Authority,Aggressiveness, and obviously penetration(but not really of the sexual nature per se), and because such things were to be encompassed by a man, since he has a penis and they "ruled"(notice the quoted past tense) for a time, it was sought for to project these symbolisms into the physical plane through the only medium for which they felt most made them a man.
Penis.
Which is really gay, cause peeps be looking at penis all the time and worshipping it and stuff :p
It's also why so many important things have phallic features/qualities, such as; the Eiffel Tower, The Statue of Liberty, The Obelisks and Pyramids of Egypt, Most buildings and houses(although has Phallic resemblances, is also womanly, in that they contain and protect life and that you enter and eventually leave them.), Subways, Airplanes, Trains, Pencils and Pens, even bottles.
Or who knows. Maybe it's a coinkydink all these things look and have similar qualities to a Penis.