The problem is that for captcha to work properly, there has to be an enormous number of possible captcha words and phrases.

If people use advertising slogans for captcha text, then the advertising execs will only be able to come up with a very limited number of slogans (they are, after all, only human and from my experience they are not the type of humans that work too hard). And if there is a limited number of captcha texts, the hackers will easily program bots to recognise them.