Everyone can have a p2p client that deals with 10kb/sec, has 100 mb storage requirements, needs 1 small cpu and 256mb ram. As you go up and up in hw requirements, cost goes up, "volunteers" go down - even if userbase goes up. As you hit datacenter level requirements the number of "volunteers" starts dropping significantly because costs start running in the 5 digit, then 6 digit category and eventually you'll be paying millions.