The type of 'depth' we require in video games is typically not the type of depth that is featured prominently in real life. For good reason! Real life depth has silly things like atoms and chromosomes and DNA and a moon that's 390,000 kilometers away! In most video games, the sky is painted to the ceiling.
When developing a world with a goal (beat the end boss; acquire certain objects; gain first place in race) any resource (time, money, processing power) that is not spent on a goal-related feature is wasted effort.
Something that matters is worth putting time into. In a racing game, time spent fine tuning cars and courses is appreciated. In a battle game, developing and balancing new abilities is key.
Meanwhile, it makes no difference if the sky is painted or actually features celestial bodies billions of lightyears away. It will not change the most optimal course on a race track, it will not make a boss easier or harder to kill.
The only exception is gimmicks. Such as racing on a beach with high and low tide, or a lunar-based boss. Naturally, gimmicks are special occasions, so spending a large amount of resources on a gimmick is a waste.