Heat. TurboCore requires a certain amount of thermal headroom to work, and games push a laptop's constrained environment to the limit.
Turbo mode ONLY works when the cores are NOT under heavy loads. With an AMD processor the TDP and core temp at any given time determines what frequency the CPU will run at. Under heavy loads the TDP is high so Turbo mode is not available. With lighter loads Turbo mode will run on until the load on all cores reach a threshold load or temp point and then the processor automatically switches to the base processor frequency or lower if the processor is overheating.