Probably one of the reasons is competition. The mobile market is saturated with various games, and to stand out, developers need to create quality products. Users are becoming more and more demanding, and great graphics and gameplay can attract more players and create a positive impression of the company. https://www.unigamesity.com/how-video-games-are-making-their-way-into-the-mobile-market/ will help you to learn more about everything and understand how everything works. 

Another reason is technological advancement. Smartphones have become much more powerful and can handle more complex graphics and computing tasks. Developers use this advantage and create games with stunning visual effects and realistic graphics, which makes the gameplay more exciting and engaging for players.