Cotton as a fabric is breathable and soft and comfortable on the skin. Your skin breathes if you must know and when there is perspiration the sweat on your skin needs to dry away for you to remain cool and to be away from skin allergies. It lets the air pass through the minuscule pores and also takes the shape of the body so fits comfortably. Cotton fabric is also soft
Synthetic fabrics do not have this facility, they are not breathable and they will stick to your body with the sweat on your skin and make you uncomfortable. It only gets sorted if there is a sweat wicking coating on the fabric wherein it takes away the sweat and throws it out of the garment also. This is mostly used in fitness wear.