A cat frequently going to the litter box but not defecating can be caused by a variety of reasons, including urinary tract infections, stones, constipation, and anxiety or behavioral problems.
First, we need to consider the cat's health. If a cat frequently goes to the litter box but doesn't urinate or defecate, it could be a sign of a urinary tract infection (UTI). UTIs are quite common in cats, especially older cats and unneutered male cats. Typical symptoms include frequent urination, urgency, painful urination, and even hematuria (blood in the urine). If your cat exhibits these symptoms, you should take it to a veterinarian as soon as possible for a urinalysis and necessary treatment. 
Secondly, urinary tract stones can also be one of the reasons why cats frequently visit the litter box. Stones can block the urethra, causing difficulty or pain when urinating. In this case, the cat may try to urinate multiple times but fail. Treatment for stones usually includes surgical removal of the stones, dietary changes to reduce the risk of stone formation, and increased water intake to help flush the urethra.
In addition, constipation can also be a reason why cats frequently go to the litter box. Constipation can be caused by a variety of factors, including improper diet, lack of exercise, hairballs, and intestinal obstruction. Cats may go in and out of the litter box repeatedly due to difficulty or pain in defecating. Treatment for constipation can include increasing the cat's fiber intake, encouraging exercise, regularly grooming the fur to reduce hairball formation, and using laxatives under the guidance of a veterinarian.
Finally, we also need to consider the cat's behavioral and psychological issues. Some cats may exhibit abnormal behavior due to anxiety, stress, or maladjustment to a new environment. For example, a cat might frequently go to the litter box as a way to relieve stress because it feels uneasy. In this case, we need to identify the cause of the cat's anxiety and take appropriate measures to help the cat return to a normal psychological state, such as providing a safe and comfortable environment, increasing interaction and playtime with the cat, etc.
