The simple answer is “because it is the rule”.

The rule says that object pronouns are always placed before the verb except in imperative affirmative sentences.

J’aime la France. → Je l’aime.
J’entends les oiseaux. → Je les entends.

An easy lesson on the subject on Bonjour de France.

There are 2 rules that answer your question.

First: subject (direct or indirect one when complement to the verb) ending with ‘e’ and placed in front of a verb starting with a vowel must be shortened.
Thus, ‘je’, ‘me’, ‘te’ and ‘se’ will become ” j’ “, ” m’ “, ” t’ ” and ” s’ ” when in front of verb starting with a vowel.
“Je aime le chocolat” is incorrect => “J’ aime le chocolat”.
“Tu me as fais peur” is incorrect => “Tu m’ a fais peur”.

Second: when you shorten a direct complement of a verb, you must replace it with an indirect complement placed just in front of this verb.
“J’aime le chocolat” => “je l’ aime”.
So, “J’aime toi” (which is a form that no one will ever used because it sounds ugly) becomes “Je t’ aime”.