The reasons why hamsters have sparse hair:

1. They are older. As hamsters age, their bodies will become weaker than before, and their skin will become saggy, leading to hair loss.

2. Long-term lack of protein. A long-term low-protein diet will lead to protein deficiency in the hamster's body, which in turn will cause hair loss in the hamster, and will also increase the risk of hair loss in the hamster.

3. Skin damage due to friction. When hamsters are kept in cages, their skin may be rubbed during daily activities, causing hair loss.

4. Seasonal hair removal. Seasonal hair loss is normal for hamsters. Affected by temperature changes in different seasons, hamsters will automatically adjust their bodies to better adapt to the ambient temperature, resulting in hair loss.

5. Licking. Hamsters lick their coat too frequently, especially during estrus, and like to lick their scent glands and reproductive organs, leading to hair loss.

6. Pregnancy. After the mother hamster is pregnant, in order to facilitate breastfeeding of the baby hamster, she will nibble off part of the hair on the abdomen and around the nipples before giving birth. This is a sign of good motherhood of the hamster.

7. Endocrine disorders. Hamsters with endocrine disorders and imbalanced hormone levels may also experience hair loss, such as excess testosterone in hamsters.