I think there's multiple factors at work.
1) It's easier to add switchology and better graphics to improve a title/over past titles from other developers that you might pick as your internal benchmark.
This is a consequence of manifest buying habits of the vast majority of customers in the target demographic. Few people are willing to look closer at a title if it looks terrible. Even if you consider good looks as a mere hygiene factor (=people may not go after the best possible looks (they do), but at least it shouldn't look terrible compared to market leaders) you can't uncouple from the trend towards flashier graphics. Theoretically all women on the 1-10 scale should be given a chance in the looks/personality matrix; in practice men are often discarding any girl that isn't at least a five (=by definition, above average). One can complain about it, or accept this reality.
2) Great gameplay is abstract, until after the purchase
When marketing is all about the art to motivate purchasing decisions, good gameplay is a highly indirect factor at best. It's more relevant for subscription models, actually. Whenever the business model is focused on one-time sales rather than recurring payments, the rational business strategy is to focus on features that are marketable, and treat engaging gameplay as a hygiene factor (see above). The saving grace for you and other game enthusiasts is that a lot of game developers aren't strictly rational in their product development strategy because they are largely enthusiasts too. The more the market drifts towards a game industry however, rational game development strategies will be rewarded better, on average.
There's more to it, but it's a Friday evening, so I'll leave it at that.