One reason is that up to quake almost every game had its own engine, hard coded by experts in pure graphics programming. After halflife and unreal the game eng

One reason is that up to quake almost every game had its own engine, hard coded by experts in pure graphics programming. After halflife and unreal the game engines became standardised (later of course came unity etc.) and the focus of selling games turned to graphic style and storyline (and often multiplayer gameplay) over the pure mechanics.