the average user doesn't host so many domains under a single hosting package and the average website doesn't use so much disk space / bandwidth.
This is a truth many hosts, and even more customers, don't seem to recognize. Even Terry's example of a resource intensive site is only 2.6 GB. As an unlimited host I can tell you with much confidence that 99% of my web sites remain under a couple GB (not counting databases and email which are on dedicated servers).
Given that and managed switches its no big deal to offer unlimited web space and bandwidth quota, especially if you do not offer unlimited sites.