You have a number of unsourced examples of people not distinguishing between characters and authors. I could give you a number of examples of attribution for characters. Justifying your claim that it is conventional to ignore characters when attributing quotes requires more than a random selection of anecdotes. When I think of famous literary quotes I do not simply think Shakespeare, I think Hamlet, or Macbeth, the distinction is important.

If you have to defend a claim to 'convention' it's not really convention.