The reasons why it is not recommended to brush a cat's teeth are as follows:
Cats don't like anyone touching their teeth. Forcibly brushing their teeth may injure their mouths or even cause them to bite or scratch their owners.
Oral care for cats doesn't necessarily require brushing their teeth. For example, you can regularly use dental wipes or dental finger cots to clean your cat's teeth.

Some veterinarians believe that cats do not need to have their teeth brushed because wild cats and other animals do not brush their teeth and do not develop dental diseases.