Ufff. You are, for once nicely and to the point idealistic. It would be awesome to have games represented from the get go as they are. I would love that. However...
Let me premise this with stating that I despise marketing as it stands today and that I do not agree with their actions. But at the same time I must admit that they know what they are doing and that their tactics work.
Simply said honesty doesn't sell, or more truthfully, honesty does not sell as well as deception and confusion. Yes, it would be great if we had, from the start, only true gameplay videos (even if that idea is stupid for variety of reasons including that game does not get final polish until late in development cycle) but job of marketing is not to tell us the truth. It's job is to sell us things. And hype and elation does that better than truth. When commercial ends and we go on with our lives content more or less goes forgotten and what is left is tone and feeling it provoked and that is what lures people into purchasing product (except for borderline fanatical hobbyist and professionals which are in majority on any gaming forum)
Let's just look at a classic example, Coca Cola. Their commercials have absolutely nothing to do with their product. And why would they? Product itself is a guilty pleasure, we know it's bad but we like the taste and sugar rush we get. Therefor their commercials go with generating positive feelings within us. Joy of friendship, music, love... Carbonated caramel soda has nothing to do with any of that but feelings linger. And look at how successful Coca Cola is.
I really have to go, I just hope I gave you people some food for thoughts.