Why aren't they even trying? Spoiling things is not marketing. Quite the opposite in fact, no matter how "cool" these characters might be.
You are very naive to think spoliing things isn't marketing. You inherently have to spoil things to promote a media. And with act 1 fully revealed, it leaves very little choice of material for Larian to choose from. I don't think they would have been able to maintain their momentum for 6 months with only monk, dragonborn and multiclassing. I actually think that what has been released isn't really groundbreaking spoilers as it is both already mentioned in game and close to what we can already play, and it makes for a nice cinematic moment that any trailer need.