Evolution favors no regeneration of missing body parts
Most animals don't have the ability to regenerate body parts. Why would evolution favor no regeneration?
If a body part were able to regenerate, than any partially amputated limb or part of a body that is deeply wounded but still fed from a nerve and blood supply would grow, which the potential that both sections would regenerate the missing piece—one from each part of the cut. Such a regenerative ability would cause the body distress because it would create a developing Siamese twin that would eventually interfere with the body's overall constitution, making it less fit. In contrast, losing the wounded body part results in a more stable constitution and one that recovers more rapidly from the wound.
If such regeneration could occur, would the wound healing reach a point that the two halves would eventually disconnect from each other, making two fully constituted and fit organisms? Cells can divide in such a manner, but to completely separate two parts of a complex body, including skin and other tissue, would require genetic code so advanced and hearty that it would itself become another form of reproduction. This doesn't occur because evolution doesn't favor this form of reproduction. Evolution of any but the smallest forms of life favors reproduction where the gene pool is constantly creating unique combinations to help ensure new forms of life evolve and to ensure that the less fit genes don't "meet their match." This constant recombination and evolution creates a grand and varied mix of life forms, which makes the "life" of a planet better able to survive if bombarded with disasters. In such a situation, genetic variety helps ensure that some forms of life would persist—the forms best suited to the new conditions. So in the grand scheme, the favoring of a varied gene pool and varied species helps ensure that some form of life would survive in a chaotic universe. Since all life on a planet is from the same gene pool, parts of the extinct life would survive within the genetic code of other forms of life—this could be termed "supra evolution." Over eons, evolution could potentially recreate an extinct species' genetic code.
That's real survival of the fittest: not survival of a species, but variation to ensure survival of elements of the gene pool itself with the potential that the extinct species could be reborn.