Phones don’t have active cooling, or just proper cooling. Heavy usage and charging will heat up the battery a lot. A warm battery at a high state of charge will deteriorate fast.

Also, they aren’t designed to last very long. One manufacturer could reduce the max voltage in the BMS (battery management system), or reduce the charging speed at high states of charges, to significantly increase the lifetime of the battery but they somehow don’t. Though you start to see some phone limiting their charge.