There is one reason for that. Two actually. Maybe three. One
is that my homemade RSS/Atom feed generator can’t convert between
human-readable dates like 20/12/11
and whatever thing is
painted grey up there. It just copies whatever is in the
\date{}
parameter.
Second reason is that there is an IETF standard for time and date. As you might have noticed, nobody follows it. Not even email users.
HOWEVER, there is a catch. It is a requirement of the Atom standard, so if you wish to have a standards-compliant program be able to read dates of your posts, which is crucial functionality for something like an RSS feed reader, then you shall use this format.
So, don’t be like me. Use a real
static site generator
like Hugo and have it do the work for you, instead of suffering like
me.