Why these systems are so cheap?

You've got to read through the hype, to what they are actually saying. "most components in brand new condition" "brand name products". No guarantee, or means to RMA. Re-conditioned components? No returns. What you see is what you get.(but a comment you might not get what you see).
It's one of those "you might be lucky, but your unlucky if you're not". Apart from Intel i5-3570K (which may, or may not be new), and some sort of Lian Li case (not necessarily the one in the picture) you really have no idea what you are getting. It's even got Windows 7 Professional 64bit!! (FOR DEMONSTRATION PURPOSES ONLY).

basically to sum it up, the parts can be refurbished/open box and you essentially are stripped out of warranty of the products. also the term "name brand" is a marketing term. Almost all computer parts will have a name.

Yeah I do know that there is no guarantee for the PC, but the price was so attractive which cost only one half or one third of original built. How can he grab these parts at those prices?

I just want to be sure that I'm buying a good one  

I would tell you that you can't be sure if you're "buying a good one!"

Also you dont know what you are saving. Sure, if you planned on spending $50 on a power supply, and they are putting in a $10 Chinese import, then you are saving money, but is that what you really want.