IP, it may be because people are proud of their work, it may be to direct people to another resource.
Many people associate this picture of a boar with me because it is used on the many profiles I have on the Internet. In a way, when people see my "zwijntje" they will associate it with me. Often pictures are shown beside threads and consequently this pig is associated with text attributed to me.
It is important to understand that attribution exists on two levels. There is the attribution that is the acknowledgment of effort and there is the attribution that is the consequence of copyright law and licensing. Many people mistake the first with the second.
Not all material in Wikipedia must be attributed, Most of the material that is being restored is in the public domain. As a consequence, copyright law does not apply to restored material and therefore there is no legal need to attribute the work to the person who restored the picture. Acknowledging this work is something that we do in our community. It is important that we do. By acknowledging our efforts in our community, a legitimate sense of pride can be felt by our contributors. People take pride in their number of articles, their featured articles, "did you know"s and featured pictures.
Pride in our achievements is provided by attributing the good work to the people who did it. Attribution in the legal sense is firmly a function of IP law, it is the kind of attribution I feel uncomfortable with.