If you'd like help with figuring out what is using all of your data please start a new post in the Tech Support section.
HughesNet can't offer unlimited plans as the system doesn't have the capacity to support it. In addition, satellite internet is the most expensive type of internet to provide, which is why it isn't cheap. It can't be compared to ground based systems. It's internet for a niche market, and that market is people who are unable to get ground based service.
The sales reps can only make a guess as to how much data you may need, and that guess is based on the averages of other users. It's ultimately up to you to know your actual data needs.