Rebyidd23: because it’s a mitzvah.
Presumably, that’s a response to “If we wouldn’t eat food for
reasons other than nourishment, we would eat the same food on Shabbos as during the week.” However, since a mitzvah is something
other than [physical] nourishment, this is not a valid response.
(Anyway, he knew that wasn’t what you meant.)
I’ve been meaning to get back to this thread, but for the moment,
Yitzchok Avinu asked Eisav for food that he enjoyed. I’m sure this
is interpreted in different ways, but “ein mikro yotzei mi’pshuto.”
(According to Rav Miller, this was in order to feel more gratitude
to him so that he could give him his brocho with more feeling.)