Because it's a rip-off. The hardware prices are twice or four times what they need to be in order to be able to make a return on your investment. It's more profitable to have a quick turn-around of capital that way. Check the facts with one of the online calculators like this one:
http://minercharts.com/
Plus, I am positive they get to hold your money, not the actual hardware, for months and play 'investment' with them.