Common Sense

What's common about it?

From Dictionary.com the first 4 definitions only.

common
com·mon   
[kom-uhn]
adjective, -er, -est, noun
–adjective
1.
belonging equally to, or shared alike by, two or more or all in question: common property; common interests.
2.
pertaining or belonging equally to an entire community, nation, or culture; public: a common language or history; a common water-supply system.
3.
joint; united: a common defense.
4.
widespread; general; ordinary: common knowledge.

If that's true why is it that less than 1 out of every 100 people I meet, encounter, or otherwise have to deal with is capable of adding 2+2 (if they even can) and doesn't have the expression of a drunken bear on their face when asked even the most simple of questions?

I call shenanigans on whoever came up with the phrase "common sense". My experiences show me that the amount of sense that is "common" is barely enough to tie one's shoe.