IP addresses are just that, addresses, not humans. Due to the shortage of available IPv4 addresses and the way that ISPs manage their networks, most IP addresses change often. For residential Internet access, this means that the IP address typically changes every few days to few weeks. Corporate static IP addresses seldom change, but still can any time the business changes service type or ISP, as each ISP has a limited number of addresses that they can use. This makes it impossible for anyone to reliably track the contribution history of any unregistered user beyond a period of a few days to a few weeks. This is one reason why unregistered users are unaccountable for their previous edits.
In short, registered accounts are required to be controlled by a single person, by policy. IP addresses, however, may be controlled by 12 different people in one year, or hundreds of different people in one day. This is because the IP address isn’t a human, it is simply a routing address, similar in some ways to your phone number. Unlike a phone number, most IP addresses aren’t assigned to individual humans, but on an “as needed” basis.