Because they sell them for more btc that they can ever mine. You should ask why people are buying them instead...
I simply don't think this statement is true. I started with 6 USB Block Erupters. They not only paid for themselves, I was able to buy 5 more and a 10 port hub with the profits. I now have 11 USB Block Erupters running at 100% profit minus electricity costs. So people are making money on these things, just not as fast as everyone would like I suspect.
Seriously? You say that my statement isn't true and you bring as an example the usb block erupters? ROI wise the worst miner in the history of bitcoin?
The only miner that noone ever made roi of?