Probably because it's a normal part of the male anatomy and they don't like being without it. For many, it's a personal statement about dissatisfaction with others -- parents, medical profession -- making surgical decisions about their body without sound medical basis. If 90% or more of European, South American and Asian men can live healthy, normal lives with intact genitalia, then that's a pretty clear indictment of the "necessity" to routinely circumcise male infants.
Why has Christina Applegate just announced that she is embarking on total breast reconstruction following her double mastectomy this month? It's not like she is going to nurse with those breasts. Probably for the same reason that tens of thousands of other women do: because it puts back the body in the form it was meant to have.
In the case of foreskin restoration, from what I understand the benefits are manifold (no pun intended): it restores confidence to the practitioner that his body regains some semblance of how nature intended it to look. Many guys (myself included) are never quite comfortable seeing penises with scars around them and lacking the only mobile part that nature put on them.
Restoration also apparently has direct physiological benefits, including more comfortable erections for many; more pleasurable sex; and for most, a marked change in the texture and color of the glans back to more like it originally was intended to be. The glans of a newborn's circumcised penis is by definition a large scar itself, since the foreskin has to be lysed from the glans to which it's fused at birth in order to cut it off. The glans heals accordingly, but its development is arrested by the surgery and the lack of development within its snug, protective mucosal sheath. The penis is far from fully developed at birth and for several years afterward, particularly the nerve endings at the business end.
I think most restorers acknowledge that many men view circumcision positively, and understand why some intact men opt to get cut. Their choice. But many more intact men prefer their penises the way they are, and a significant percentage of circumcised men wish they weren't. It makes at least as much sense to want back what was taken away as to want to cut off what nature put there.