There's definitely little constraint in what you can achieve. But a lot of low effort games are made by only setting up simple game logic and creating a few models and textures. This means they all use the same default lighting calculations, post processing steps, etc. That gives them a similar look.

Contrast this with more ambitious games and you'll usually end up with an array of custom shaders, a stack of custom post effects, etc. That really lets you define a look that's all your own.