First, malnutrition. If the cat's hair loss is not caused by licking itself and there are no abnormalities in the skin where the hair has fallen out, then it may be due to malnutrition causing sparse hair. This is common in cats that have been fed a monotonous diet for a long time or are fed cheap cat food, which can lead to a deficiency of certain trace elements in the cat's body. 
Secondly, endocrine disorders can cause hormonal imbalances in cats. Thyroid disease or excessive secretion of male hormones in unneutered cats can lead to endocrine disorders, often resulting in symmetrical hair loss.
Third, if the skin in the area where the cat is losing hair has dandruff, redness, swelling, or itching, it may be a symptom of a skin disease caused by parasites, fungi, bacteria, or other pathogens. It is recommended that pet owners take their cats to a veterinary clinic for examination as soon as possible to determine the specific cause and take appropriate treatment measures to prevent the hair loss from worsening.
