There may be several reasons. The main reason is that sit-ups do not involve abdominal muscles (rectus abdominis), but the anterior thigh muscles (quadriceps femoris) and iliopsoas muscles. If your hip flexion ability is insufficient, then sit-ups will involve your abdomen to compensate for your hip flexion ability. Many people go to the gym to practice abdominal muscles, and I would recommend them to stimulate belly rolls instead of sit-ups.

Among the iliopsoas muscles, psoas major originates from thoracic vertebra 12 and lumbar vertebra 1~5, and iliopsoas muscle ends in iliac fossa. The stopping point of iliopsoas muscle is femoral trochanter. Therefore, strengthening iliopsoas muscle can easily lead to trunk flexion and pelvis forward tilt. For modern sedentary people, iliopsoas is one of the muscles that should not be "over-strengthened". This is another important reason why I don't advocate doing sit-ups.

Excessive burden will cause physical injury or discomfort to the spine, transient cerebral ischemia, or neck compression and hypoxia caused by insufficient movement will cause headaches.