Humans aren't made that way. We don't have to have limbs that can regrow because we don't have any natural predators, so we don't need camouflage.

In all mammals, limbs development from the "limb bud" is controlled by a series of genes which are only active during embryological development. At the point that the complete limb is present, these genes are shut off, and a new series of genes take over which controls cell division in a slightly different way, to expand and lengthen the areas of differentiated cells which are already present. These control growth through infancy and childhood and into adolescence. However, around the time that the body's hormones signal maturity, even these genes are shut down, and then the only genes available to direct growth are the ones that we normally use for healing wounds. These are capable of sealing up cuts and growing new skin, repairing damaged muscle cells, re-establishing intact bone across a break, and things like that -- but we no longer have the pluripotent undifferentiated stem cells which would move out to form all the tissues of a limb from new, and the genes which would control and direct such growth have been permanently silenced. Genes are silenced by the addition of a methyl group to the start of a gene, which blocks the gene from being transcribed. There are actually genetic disorders where these genes are not silenced properly and are overactive; however, this does not result in the ability to regrow lost limbs, it results in some fairly major deformities where limb growth is triggered in areas of the body which shouldn't have limbs. In amphibians and the few reptiles which have the ability to regrow lost limbs, there appears to be a way for the genes controlling development and cell differentiation to become selectively "un-silenced" in a process which is clearly under control. However, we do not yet have a good understanding of how that works.