MTU of the Internet?

Several people have noted to the Microsoft Support and Product groups that they want the Windows 95 PPP MTU to be set to 576 (down from 1500). this change is in Windows 98. The reason for this change cited by many customers is that many ISPs have 576 MTUs set "inside" their networks and packets get fragmented. How prevelant is this fragmentation and how prevelant is this MTU in ISPs? Are there statistics that can be shared on how much traffic they see on their networks that are IP fragments? Why do ISPs set their MTUs to 576 instead of ~1500 or even ~4K? thanks in advance for your responses, peter