I'm guessing from a business perspective why it makes sense for them to do.
It makes them enough money to continue what they are doing. Putting more effort into it may get them some more viewers, but not enough viewers to justify the cost (or at least, that's the business risk they are placing their low-stakes bets on). People who care enough to get mad at this video are a vocal minority. If that's not the case, then I'm wrong, and Polygon is wrong, and they will lose viewership for it. My guess is that the people who are upset about this either don't care about Polygon in the first place, or it's not enough people for Polygon to care about losing. But I could be wrong.
Are you arguing that the visuals, narrative, and audio have nothing to do with Doom's appeal? Sure, the gameplay is a large part of the appeal, but if I'm looking to see if the new Doom game is good, gameplay is not the ONLY part of the appeal. What if I just want to see if the tone is goofy or self-serious? What if I want to hear the music and hear if it's reminiscent of the original? What if I just want to see how fast the movement speed is, to see if it's similar in that way to the original? None of these rely on someone playing well.