Why do the French say 'je t'aime' and not 'j'aime toi'?

Answer 1

In some latin languages, pronouns are put in front of the verb when used to express the object which is considered

Compare with italian or spanish: ti amo, te quiero

Answer 2

In French, "je t'aime" directly translates to "I love you" where "je" means "I," "t'" is a contracted form of "te" which means "you," and "aime" means "love." Saying "j'aime toi" would be grammatically incorrect in French as "toi" is already implied by the use of the direct object pronoun "te" in the verb "aime."