Humans are supposedly born with inalienable rights - life, liberty, the pursuit of happiness. Our societies in the West are built upon this shared foundation of rights. To deprive any adults of the ability to procreate would be considered barbaric, as they want to pursue the creation of "life" with their liberty, and to support their own happiness.

Meanwhile, nature is just raw competition, we're used to talking about genetic evolution in natural populations. Natural creatures aren't deemed to have any right to their own life, liberty, or pursuit of happiness, or at least not by most humans outside environmentalists.