Clients with no verified payment system

I'm both a client and a freelancer. As a client I have a verified payment system that I use when I outsource my work. I've a question. Why does oDesk not make it mandatory for every client posting jobs to first have their payment system verified? I receive invites to apply almost everyday from clients who have no verified payment system, and it is irritating to see that. What good does it do to contractors to apply for a job where the client has no verified payment system? Of course it's from new clients on oDesk, but it would be wise to have clients verify their payment system before posting jobs. It would reduce the number of scammers on oDesk.