There is real world reason why everyone does it.
Graphics are usually one of the last things that get finished in games. There is no sense spending hundreds of hours and thousands if dollars to make character models and levels pretty, when there are bound to be changes in both.
Teaser trailers basic gist is to show people "hey we are making this, follow us for more", cinematic trailers are world building and showing the overall theme. Also if the public has bad reaction they still have time to make changes to the game.
Down the line when the devs are confident that aspects of the game wont go trough any more major iterations they can start to show gameplay.
Inherently all trailers are just marketing and the very fact you are commenting something here says that they have succeeded.
Moaning about cinematic trailers is like asking a cheff to show the steak while the animal is still grasing on the meadow.
And im giving you a context "why everyone does it".
There is also plenty of other benefits in releasing cinematic trailer. First thing of course is to get knowledge of the game out there and create intrests, but it also:
Helps devs to gauge the intrest in their game, See how people react and if necessary change things early in the production, If company is in stock market, it necessary to make clear to the public they are doing something, If they have financial backers its easy to give them reassurences by telling how many times the trailer has been watched and commented. Especially important if they have nothing else going on that quater or financial year, Youtube statistics also give devs insight to their fanbase as they see who are the people watching it, and that helps them gauge things like need for localization
The part about moanin isint necessary about you, but more general thing. Sorry if it felt like attack towards you.