The reason why TV ads are so good at creating demand, I think, is because it’s in a medium aligned with entertainment. You’re passively sitting back, letting the TV take you where it wants, the ads act as mini stories that you’re already expecting to see.
The internet is much more of an active, informational and communication-driven medium. You go to the internet to find things out just as much, if not more, than for entertainment. This is why display ads generally suck – they don’t fulfill demand, and if they do any creating of demand it’s minimal because the user is not in entertainment mode, they are in research/communication/info gathering mode.
Content that is interesting/informational/helpful/entertaining is a little closer to nirvana – it can fulfill demand: people looking for answers can find them in your content, which hopefully refers them to purchase from you, and it can create demand: the content can be convincing enough to create a new need in their mind to motivate them to buy something. But quality content doesn’t scale in a linear proportion like paying for ads does and it’s hard to always be original.
We will reach advertising nirvana (maybe) once the internet more closely resembles television so that it becomes more of a passive medium while retaining it’s active capabilities, and some new forms of advertising are invented that better facilitate the two.