Why do people give stats and not back them with a source?

Question

Is it just to purposely misinform and delude others.

Answer 1

Because if they gave you a source, you could look it up then call them liars.

Answer 2

There are many reasons. Sometimes it is "puffery", totally arbitrary claim with no backing numbers. sometimes they may recall the numbers from a report on the radio or TV or from a magazine or news article, but not remember the source.

And often, they are repeating someone elses puffery.    

Answer 3

Providing stats makes an argument sound more scientific.  The majority of people outside the academic realm do not bother with the source, because many do not have the capability (or conscientiousness) to assess the verity of a source - they just assume it is true.  Graduate schools stress the importance of questioning sources when conducting research, but then again, most people don't go to grad school.