Because they are the ones distributing the binaries. Though I agree it
does not make much difference here - but in general how do we know if
the official sources contain all the modifications needed to make the
software build correctly? It is safest if the code is available, at
least in principle, from a single source (i.e. the same one that
supplied the binaries).
It also avoids the following situations:
A hobbyist writes some code, and publishes it on his personal website.
A large corporation then distributes binaries with their product, and
refers their users to the author's website for the source. The author's
website either goes down or gets shut down for drastically exceeding its
bandwidth quota.
A less common issue is that the site hosting the source may be less
accessible than the one hosting the binaries. E.g. the source may only be
available by CVS (which may be a problem if you're behind a firewall or
proxy which only allows web and email), or it may be on a shared community
site which emphasises free speech, and thus finds itself on the wrong side
of filtering proxies.
Hence the GPL's requirement that "equivalent" access means that the source
must be available from "the same place" as the binaries.