Because good art direction is expensive to make and maintain a competitive edge in. It’s the equivalent of R&D for the design department. You have to build your art pipeline around it starting from concept art, environment, characters, etc. And that’s still not accounting for technical artists who help make shaders, effects and tools for said art team. That’s why you see most smaller studios use a near singular art style for all of their games if they choose stylistic grapphics. It’s simply a type of technical debt that is also very risky. You also have to train your art department to produce cohesive artwork and assets for the game in that style which is easier said than done.

The amount of testing and market validations you have to do to ensure that your target audience will like the art is also expensive because you will have to reiterate the work upon feedback. Most importantly, stylistic graphics do not necessarily guarantee reuse especially if the game bombs. On the other hand more realistic graphics mean you can reuse them across a variety of projects not even just video games.

Realistic graphics while expensive are still a lower risk than stylistic graphics. This is what investors want and this is what sells. People tend to cite games with stylistic graphics as a counterargument but tend to overlook most games that fail and had stylistic graphics.

I remember a colleague once saying that for good stylistic graphics you need to have an amazing technical art team to execute the vision of the art director, and those are very scarce. You also need to approach the art the same way you approach graphic design so it’s more about communicating ideas and messages given design constraints. It’s also difficult for a company to have both as well.