Human beings. We need food & shelter. After that, it really are just wants. Some want fancier houses. Some want cars. Some want exquisite wines, watches, holidays etc.. Our wants are unlimited. Our wants always exceed our needs. Why? Consumerism. Greed.

So bankers manufacture another product as demanded by society- CDOs. Speculators hoping to make a quick buck out of a spiralling property market. Mortgage houses giving them cheap loans. Investment banks helping to collaterise the loans. Ultimately the greedy investors themselves who demand such "high-yield" products. What's the cause? It is not the banks. It is Greed.

Bankers are just manufacturing products that society demands. The better you are at it, the better you get paid.