The number of games in general that use in-game screenshots/models for cover art, whether AAA, indie or in-between is in the minority. Games typically use either some separate render using assets different in quality/location/arrangement than what players will actually experience in-game (to varying degress), or a different style entirely. Such graphics are fundamentally there to get your attention and interest.

Especially with pixel art one of the obstacles to things like banner graphics is how vastly different the scalings will be for any given site/context. Depending on the resolution of the source graphic such blocky, zoomed in pixels can easily come out strangely anti-aliased or blurry as a banner scales up/down which is another thing to consider.

Contrast to a well-made, high resolution illustration which can easily be cropped, arranged and scaled for different contexts and it's easy to see why these are frequently used. Some are works are art in their own right, as we've seen for various games in previous eras (one of my favorites being the Japan Prince of Persia cover for the Famicom), and can be seen adorning a vast number of titles on GOG's own game page backgrounds.

Fwiw in the case of this game I'm not one that would actually like a game using the high-detail illustrative style of the banner as *far* too often games that use such renderings utilize shortcuts to reduce development complexity, such as animation marionetting (eg: the 'Flash-look'), or very stilted movement frames. It's due to this I usually expect such promotional graphics to be stylized and not representative (or hope so, usually).