[Avodah] Why didn't the other nations accept the Torah?

[Marty Bluke via Avodah]

There is a famous Medrash that before offering the Torah to the Jewish
people Hashem offered it to the other nations of the world.

 "First He approached the children of Esav and asked them, "Do you wish to
accept the Torah?" They replied, "What is written in it?" "Do not murder."
They said, "...Our father  was assured  that, 'By your sword will you live
(Bereishit 27:40)!'"

Next Hashem went to the children of Ammon and Moav, and asked, "Do you wish
to accept the Torah?" They asked, "What is written in it?" "Do not commit
sexual immorality." They responded, "Master of the Universe, our very
existence is based on an immoral act!"

Hashem then went to the children of Yishmael, and asked them, "Do you wish
to accept the Torah?" They asked, "What is written in it?" "Do not steal."
They responded, "Master of the Universe, the essence of our father was to
be a bandit, as it is written, 'And he will be a man of the wild; his hand
will be in all...'" (Bereishit 16:12)."
Sifri, Ve'Zot HaBracha #343

However, the Medrash has an obvious question. All of the examples of Torah
prohibitions given by the Medrash, are also prohibited by the 7 mitzvos of
Bnei Noach. The Bnei Eisav were already prohibited from murder so how would
accepting the Torah make things worse for them? The same goes for arayos
and stealing, these were already prohibited to them anyway so why does the
medrash specifically pick these as examples for Torah prohibitions?
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