Because you have to use articles with nouns in English.
But in fact if 'Kedi bahçenin içinde.' someone said, i would ask 'Hangi kedi?(Which cat?)' because 'kedi' alone itself indicates that there is a cat meantioned before. If one was supposed to tell me that there is a random cat in the garden, 'Bir kedi bahçede.(A cat is in the garden.)' or 'Bahçede bir kedi var.(There is a cat in the garden.)' that person would say.