Why is it that Ubuntu comes in so many varieties which are essential the same thing except for the desktop environment and packages, both of which are easily downloadable? Is there more of a difference between ubuntu distros?

Because it can be...
Because some people are way too lazy to download KDE and amaroK, or some bible software. Downloading the first ISO is already a big effort you know...
Actually Ubuntu itself is already just a collection of software which can be downloaded and installed à la LFS.
The only added value of a distro is packaging. kubuntu and ubuntu are just different packagings for different tastes. There are many distros because there are many tastes and building a distro is relatively easy; so why not make a distro from ubuntu with some extra packages and some removed? Some people will be happy for less effort.

It is a lot easier to make a name for yourself by mixing a already developed desktop with a core Linux distro by another.

because some people want KDE or Gnome and might not use that other so it would be easier to get KDE lets say and not waste HDD space by getting both

That seems like a pretty shallow reason for investing into and creating a distribution.

There is no other difference. They are intended for different needs:
some users prefer gnome, some kde
some users have an old machine (<256MB memory) so prefer xfce for performance reasons