That's an easy one: your product is good only for some people, who happen to have the problem you're solving. You could tell everyone about it, but you'd reach a lot of people for whom wasting time hearing about your product is a net negative, and it's entirely possible that the overall value to other people's lives of advertising in that way would be negative as well. Of course you could try more targeted advertising to mitigate the negative externalities, but that carries its own costs in terms of invasion of privacy, etc.

So yes, it's certainly possible that advertising raises the overall well-being of the recipients, but it's by no means a given.