You basically said it all.

People also have to understand that at the time the initial trailers are shown, the game isn't fully compete. So especially if a game is open world, it will be missing npcs and environmental elements such as trees or buildings and some areas of the map might not even be in the game yet. So at the time the game would look and can fully run on the hardware at it's current state, but then add those missing factors I mentioned previously and then suddenly you'll have to start scaling back things like graphics or resolution to get the game at a playable state with core features in tact on current hardware.

A lot of gamers don't understand this so they see it as being deceived.

You're correct. Most Devs are extremely ambitious and want to shoot above a console, or even PCs pay grade.Well, I have worked with 3d application as such as Mya/3d max and one thing I always notice I do(as well as some of my classmate) we tend to add lots of effects to a character model or an environment because it looks better. But the reality is when you start to render a scene and it doesn't work you have to cut out lots things like lighting, fog effects, reflections, refractions, etc.

Two people downvoted you, this makes you wrong!

Clearly, it is because they want to suck us in then go "HA HA" and render the graphics worse, because "why not" right?