Environment 3D models in mobile games are simple mainly because mobile devices have limited processing power and memory—complex models would cause lag, overheating, or fast battery drain, making games unplayable.
This simplicity balances visual quality with smooth performance for most phones, plus it keeps file sizes small so users with limited storage or data can install games easily.
If you want more detailed environments, try games optimized for high-end devices (like Snapdragon 8 Gen 2 or iPhone 15 Pro)—their stronger hardware handles complex 3D models without performance issues.