Short answer
Even with a healthy diet, your body may not get enough. Studies show nutrient content in food has dropped by up to 14% due to changes in modern agriculture. Creatine is found mostly in meat and fish — if you don't eat enough, your natural stores are low.
Notes
A supplement doesn't compensate for a poor diet — it adds what a "balanced diet" can no longer fully provide.
Modern diets, reduced meat consumption, and nutrient decline mean most women over 30 are running on a fraction of the creatine their cells actually need.