In my opinion, one of the reasons is obviously because of the costs, as you say most NFT games are simple images and transitions. Because within the programming field it is much "easier" to program that than to create an open world, for example.

Being NFT games the main goal of users when playing them will not be fun but to generate revenue, and as developers know it, they prefer to save production costs and avoid creating complex mechanics, story and everything that can contain a video game.