Why IS common sense so UNcommon?

On writing we are told to not use I very often. 
It makes the writing seem self involved and they are right. 
This however is an I moment. I don't understand why people are so adverse to using simple knowledge. 
I can not get my mind around the fact that people will actually make a task harder rather than employee a little common reasoning. 
Here is an example that comes to mind. You live in Florida. 
A state with no real change in seasons. 
The only change that takes place yearly is the temperatures go from hot to hotter. 
Knowing all of these weather traits people still insist on leaving their pets and their children in vehicles in the Florida heat. 
Now there are those who will argue that there are many reasons to avoid this error in good sense, but let us stick to this one for now. 
It is just to hard to imagine that people do not, will not, and do not, get it. 
It is only common sense.