Anyone can create a Steem account, but the reason no one else has really setup a site or way to do it is because you have to add STEEM to those new accounts to create them. Each account requires either an initial deposit or delegation of SP. Steemit.com does all of them for the most part because they're the ones sitting on millions of Steem and can afford to do it.

If steemit.com goes down, you'd still be able to use busy.org or chainbb to view the content and perform some actions, and you canalways use the cli_wallet or Vessel (the desktop wallet I built) to manage your funds.

I don't use steemit.com anymore for the most part :)