I agree with you that the Intro trailer is very high quality, and visually more pleasing than the gameplay. Unfortunately, there is no easy way to replicate this in their gameplay engine.
The intro movie was outsourced to Unit Image to make a movie-quality sequence. They would probably have used a completely different set of tools to Larian, and would have pre-rendered and stored the image sequence. Larian might have provided the high-poly geometry for the models, but the materials and lighting technology are clearly different, and likely the whole sequence was hand-animated and illuminated to display everything to best effect.
The gamplay engine must perform the rendering in real-time on moderate hardware with whatever data is thrown at it. Lower-poly models must be used, with the detail simulated through techniques such as displacement/parallax/normal mapping, lighting and shadow models must be simplified, and material interactions with light reduced in complexity. To mitigate the reduction in visual quality, and simulate volumetric behaviour of fluids a selection of post-processing is applied to each frame. The result will never be as good.
Larian clearly had engine tech and some generic assets from D:OS2 that form the basis of BG3, so the colours reflect that to some extent, as well as using published images for 5e. I would definitely prefer the intro look and feel, of course, but at this point I'll just be happy for it not to white-out as much as this teaser trailer.