Why it is practically impossible to build Commonsense KBs -

Consider any 'incident'/'event' in everyday life. From it, at least one inference can be drawn. For example, consider the incident 'A child got a new bicycle today'. From this, one inference that can be drawn is - So he must be happy today! Now, this inference-drawing is an exhibition of common-sense. This is based on a piece of commonsense knowledge - when children get new things, they become happy. 

So, corresponding to every possible KIND OF event/incident in real life, there will be at least one inference possible (exhibition of common-sense), which in turn is CORRESPONDINGLY BASED UPON A PIECE OF COMMONSENSE KNOWLEDGE. So, corresponding to every possible kind of event/incident in real life, there will be atleast one piece of commonsense knowledge.

Now, the number of different possible kinds of incidents/events in real life is practically infinite. Just imagine - 'I deleted an email'. 'I painted a chair'. 'I arrived at the party'......  And rarely any piece of commonsense knowledge will repeat. So the number of pieces of commonsense knowledge responsible for the exhibition of all instances of common-sense is an impractical number!