On my current hosting setup I have 5 domains each with a different user name. But I can still access all the domains from one FTP connection and I only have one control panel account.
I suppose it depends how the hosts setup their system.
Why? The answer to that can be given in a single word. Security.
To put it longer, if one of your sites gets compromised then the hacker can only gain access to files and content from the one user account, if you have several accounts every site is isolated. If you run everything under one master account, then you have problems because once in, the hacker has access to everything.
Yes - security is thing no. 1. Another thing - separate FTP accounts can be given to different people, who don’t know each other, to administer their own web sites without having access to other sites.
Sometimes, when I have a few small web sites that are custom made I put them under one account for convenience. But whenever there is something more serious to host or a web site made with one of the often hacked systems like Joomla or Wordpress - I always create a separate account for them.