Let me first say (1) in "it is me.", "Me" is the dative, not the accusative,
S. Nom. ic, gen. min, dative me, accusative mec.
Pl. Nom. we, gen. user, dative us, accusative usic.
(2) The answer is "because the French say it that way."
English grammar usually agrees with German.
The war for It is I as opposed to It is me, was probably lost in the 1990's when Quirk agreed that "It is me." is correct. It had been going on for a long time. In 1877, Ebenezer Cobham Brewer wrote in the entry on “me” in “Errors of Speech and of Spelling, Volume 1”
Me, objective of ‘I’
S.Nominative I, possessive. mine, objective me;
Plu. Nominative we, possessive ours, objective us.
‘Me” is used after the verb To be, and after the words than, but, like, and as, with such pertinacity it is at least doubtful whether it is not correct. C’est moi is the French Idiom, not C'est je, and It is me is far more common than It is I. (“Me” is dative not accusative case.)
So again, the French say il est plus riche que moi, or plus riche que je ne suis, “more rich than me,” or “more rich than I am.”
This is the style adopted by English.
(At this point, you may wish to have a look at the “official” French dictionary, Larousse for "Moi":
Moi (My translation)
(• This form used in all the functions and positions of personal stressed pronouns (also known as disjunctive pronouns and emphatic) pronouns,
(i) in apposition to ‘I’ or ‘myself’,
(ii) as the subject of an infinitive or participle,
(iii) after a preposition,
(iv) after it is,
(v) in phrases, and
(vi) as postposed complement of a imperative):
“Me departed, there will be no one left. (Compare with “With me gone/With my departure, there will be no one left.”)
“You think like me.”
• Moi is also used to indicate the interest taken by someone who gives an imperative: Look at me how he is dressed. (This is in the sense of “[I have looked, now you] look, how he is dressed” this construction does not exist in English – we would say “I say! Look how he is dressed.”)
• Moi can be reinforced by “-self”, to which it is linked by a hyphen: I astonished myself.
Brewer continues with the answer:
It is by no means certain that these Gallicisms should be abolished, but grammarians stoutly resist them, and the tendency of the educated classes is more and more in their disfavour.
(The following are correct.)
You did not suspect it to be me. You did not know it was me.
He likes you better than me (than he likes me).
(It Is quite certain that we did not use the object me after the verb to be before the Conquest. We said ic sylf hit eom (I self it am – it is myself), and Chaucer frequently writes it am I, but never it am me.